Add library_name configuration property to PlexUpdate plugin (fixes #1572)

[jackwilsdon]

Adds a configuration property to allow the use of libraries that are not named Music. This fixes #1572.

plexupdate:
  library_name: Music (new)

[jackwilsdon]

Should there be a test for library_name (maybe pass a name to add_response_get_music_section?) or is it OK to just fix the failing tests (due to changing the signature) and leave it at that?

[jackwilsdon]

Is there a reason you are not using .get() in your tests when retrieving configuration properties? I have to use self.config['plex']['library_name'].get() to get the Unicode value of it, otherwise the comparison fails (comparing a string and a beets.util.confit.Subview).

[sampsyo]

Looks awesome! Thanks! ✨ As always, a test for the new functionality is helpful if it's not too difficult (but don't worry if it looks like more trouble than it's worth). Would you mind adding a changelog entry too?

And yes, it looks like those tests should be using .get(). Seems like a simple oversight.

[jackwilsdon]

Should I create a new PR using .get() for all tests? Or should it be left as-is for now?

I'll add the changelog entry in a moment 😄.

[sampsyo]

Amending this PR will be fine in this case.

[jackwilsdon]

Everything should be sorted now.

[sampsyo]

Awesome! Thanks again!