Support global hubs

[jjlawren]

Description

Exposes global hubs in addition to the existing LibrarySection-level hubs.

The sectionID and identifier parameters can limit the results returned by the Plex server.

sectionID can be a LibrarySection ID, a list of those IDs, or the special "playlists" value.
Examples: 3, [1, 2, 8, "playlists"], "playlists"

identifier is a Hub.hubIdentifier or a list of them.
Examples: 'home.continue', ['home.movies.recent', 'home.ondeck']

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods
• I have added tests when applicable