Shuffles a bit of stuff out of App::WRT::Renderer into App::WRT, including
the method now called get_all_source_files() and a new one called
get_all_day_entries() that returns a sorted listed of days.

Uses state variables to cache the get_all_source_files() data, as well
as month_before() data (this was previously memoized inside a closure).

App::WRT::feed_print() now takes a list of entries instead of a month to
render the feed for.

App::WRT::feed_print_latest() gets a feed with most recent feed_length
entries.

This is based on a recommendation from CPANTS:

https://cpants.cpanauthors.org/release/BRENNEN/App-WRT-v4.0.1

> Add a 'repository' resource to the META.yml via 'meta_add' accessor
> (for Module::Build) or META_ADD parameter (for ExtUtils::MakeMaker).

It's shorter and the letters are all right there on the keyboard.

- strip HTML::Defang
- get rid of old wala markup code
- use Discount for both markup modules (with NOHTML flag in wala)

...also test that it'll render a snippet.

I don't really remember what shape any of this is in.  Just sort of poking
at it.