New Plugin: Titlecase

[henry-oberholtzer]

This plugin aims to address the shortcomings of the %title function, as brought up in issues #152, #3298 and an initial look to improvement with #3411. It supplies a new string format command, %titlecase which doesn't interfere with any prior expected behavior of the %title format command.

It also adds the ability to apply titlecase logic to metadata fields that a user selects, which is useful if you, like me, are looking for stylistic consistency and the minor stylistic differences between Musizbrainz, Discogs, Deezer etc, with title case are slightly infuriating.

This will add an optional dependency of titlecase, which allows the titlecase core logic to be externally maintained.

If there's not enough draw to have this as a core plugin, I can also spin this into an independent one, but it seemed like a recurring theme that the %title string format didn't really behave as expected, and I wanted my metadata to match too.

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. - Not 100% coverage, but didn't see a lot of other plugins with testing for import stages.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[henry-oberholtzer]

Could not get mypy to play nice with this type here - any suggestions appreciated.