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".

- move get_date() from App::WRT::Util to App::WRT::Date
- make App::WRT::FileIO and ::Util less redundant
- add App::WRT::Mock::FileIO for faking writes in tests
- make bin/wrt-render-all testable, add t/bin-wrt-render-all.t