App-WRT-v6.0.0

Changes for version v6.0.0 - 2019-05-06

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