Add ability to retrieve a list of items and collection object from media tags

[JonnyWong16]

Description

• Add ability to retrieve a list of items from media tags.

>>> show = plex.library.section("TV Shows").get("Game of Thrones")
>>> show.reload()
>>> show.genres[0].items()  # Return all items in the library with the same genre

• Add ability to retrieve the full plexapi.collection.Collection object from a plexapi.media.Collection tag object.

>>> show = plex.library.section("TV Shows").get("Game of Thrones")
>>> collection_tag = show.collections[0]  # MediaTag collection object
>>> collection = collection_tag.collection()  # Full Collection object

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• 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