You can add a filters.prop in an entry, containing a list of filters to
apply which will be pulled out of the guts of wrt.  I'm not sure if this
is a good idea or not, to be honest.  It's elegant enough, but it feels
a bit obscure.

I'm also not sure if it should accommodate the user writing custom filters
that can sit in the wrt repo, but it kind of feels like that would be
more in the spirit of things...

- adds an example "flat" site with, for now, a single index in root for
    testing purposes
  - wrt-render-all handles default entry instead of just using "new"
  - EntryStore::all_renderable() correctly filters out paths ending in
    "/index" instead of "index", which if nothing else corrects a subtle
    bug that would have gotten in the way of naming something "windex"
  - WRT::handle() will return the contents of a root-level "index"

This entails a bunch of stuff, and I'm not sure how good an idea a lot of
it is.

  - Adds a new tagging format for properties:

      tag.foo.bar.prop

    Where `foo` and `bar` are treated as elements of a path.  Thus if
    `archives/2019/10/5/tag.topics.technical.prop` exists, then
    the entry at `/topics/technical` will link back to `/2019/10/5`.

  - Adds "virtual" entries - if tags link to entries that don't exist,
    they're implicitly created.

  - Adds a concept of metadata distinct from (but about) entry text, and
    caches this for every entry.

  - Adds caching of child entries to EntryStore.

  - Migrates `get_sub_entries()` and `subentry_expr` into EntryStore.

There are XXX comments littered all over the place at this point.  Stuff
will definitely change before a release.

This stashes the HTML version of every entry in memory and uses
Mojo::DOM to extract headers from the markup for use as titles.
Titles are displayed in $self->{page_navigation}, now available
inside templates as ${page_navigation}.

In order to keep Mojo::DOM from choking on other input, it uses the open
pragma to open everything as UTF-8 by default, which also eliminates a
whole class of character encoding bugs and removes some fiddling with
Encode::decode() from feed_print().

This is all obviously a more memory-intensive, but caching the markup
turns out to have the side effect of making it much faster to render
even a large site, probably as much as anything because the HTML in
question is only getting generated once per entry instead of
(potentially) 2-3 times.

This commit isn't very atomic.  In the process of roughing it out and
testing it, I made a small pile of minor but potentially breaking
changes:

  - Removed entry_map from settings and hardcoded handling of various
    types of entry as some if-statements instead.

  - Removed embedded_perl flag in settings - was always turned on in
    practice, and wasn't very coherent since templating would have
    broken without it.

  - bin/wrt-display - now handles the "feed" alias correctly

  - EntryStore: now supports retrieving values for properties with
    prop_value() - this isn't currently used, but it seems like a
    reasonable extension of the property idea.

  - Added `wrt ls --with-titles`.

  - Added dependency versions to Build.PL.

  - Refactored Markup's line_parse() a little.

  - Refactored some tests to give cleaner / more useful output.

  - Renamed default template file to "default".

Including:

  - children() - get immediate children of entry
  - parent($entry) - get immediate parent of entry
  - days_for($container) - get days in a month or year
  - months_for($year) - get months contained in a year
  - basename($entry) - get basename of an entry

These are used in App::WRT::year() and App::WRT::month(), replacing
some dir_list() invocations and at least somewhat simplifying the code.

This commit is something of a hairball, the result of evenings-and-weekends
hacking building up a set of changes that got out of hand in parallel.
If I had the energy to spare, I would break it apart into
semantically-related changes, but I don't - and I suppose all this crap
being rolled together is at least reflective of how the code was
written.

These changes are really half-finished, at best.  Eventual goals:

  - App::WRT shouldn't directly touch the filesystem
  - App::WRT::EntryStore should model the entry archive completely
  - App::WRT::Renderer should say what to write to the publication
    directory
  - This one's a maybe: Filesystem interaction should pass through
    App::WRT::FileIO or something like it so that EntryStore and Renderer
    can be more usefully tested, with mocked writes (maybe)

I do think this represents an inflection point in the long, silly life of
this program:  It includes a handful of new tests, and a number of the
code changes were in turn easy to make because the test suite begins to
model the code in a useful way.  It's less and less necessary to run wrt
against the p1k3.com archives to be sure that I haven't trashed something.

Breaking changes to note:

  - Will no longer render HTML for nonexistent entries
  - Months and years which are flatfiles or contain an index are handled
    differently, albeit less brokenly
  - EntryStore includes index files in its overall list of entries
    (this seems to break less than I thought), which trickles out to
    bin/wrt-ls

Overall changes herein:

  - App::WRT::Date
    - Move month_name() in here from App::WRT, add tests.

  - App::WRT::EntryStore
    - Hash file types for entries (directory or flatfile)
    - Use keys of file type hash for complete list of entries.
    - has_prop($entry, $property)
    - is_dir($entry), is_file($entry), is_extant($entry)
    - parent_of($entry)
    - has_index($entry)
    - Make EntryStore cache whether a file is a flatfile or a directory, as
      well as its existence, in a single hash.
    - Include index flatfiles in @source_files for use by has_index()
    - Various tests.

  - App::WRT::FileIO
    - Still duplicates a bunch of shit from Util, so that needs sorted.

  - App::WRT::Renderer
    - Convert to a proper class.
    - Add experimental FileIO class to use in Renderer (imperfect,
      tricky, still thinking about this).  The idea is to separate out the
      concerns of reading and writing the filesystem.

  - App::WRT
    - Refactor display() and improve tests
      - Use "@entries" instead of "@options" for clarity
      - Handle entry names that might evaluate as false
      - Test running display() without any params
    - Rename expand_option() -> expand_alias(), refactor
    - Use EntryStore::has_prop() to detect wrt-noexpand.prop
    - year(), month(), entry() partially rewritten to use EntryStore
    - year() should handle months which are a flatfile
    - Refactor icon_markup() to use is_file() / is_dir() / is_extant(),
      add tests.
    - Add subtitle to feeds

  - bin/wrt-ls is now a "modulino" with tests

  - bin/display errors on non-existent entries

  - Build.PL
    - Remove bogus XML::Feed dependency