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.

It's weird to have this hardcoded, and there's
context on each entry anyway.

The default was `$0?`, which I think was probably a holdover from this
being invoked as a CGI script.  `$0` would give the name of the current
script, so you'd wind up with `display.cgi?2008` or what-have-you.

Adds a wrt-init kind of on the model of git init for starting a stub
directory with wrt.json, archives/, publish/, and templates/.

Also starts to tweak POD a bit.

I went and read:

https://pause.perl.org/pause/authenquery?ACTION=pause_namingmodules

Realized I should probably not name this something with an inscrutable
top-level namespace.  WRT is still inscrutable, I guess, but at least
it's now under App.

- Renames all modules, fixes tests (such as they are)
- Zaps old t/validate/ stuff