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