Support local media (subtitles, channel profile pic)

[kiyoon]

Make it able to load extra local assets supported by Plex.

For example:

• Now it can load subtitles (#11)
• Load channel profile photos (add show.jpg in your channel folder) (#10, #16)
• Banner images
• Theme song
• Fix #20

[lamusmaser]

It should be noted that this may not resolve the issue if you are affected by this bug: https://forums.plex.tv/t/bug-in-legacy-localmedia-agent-prevent-year-based-seasons-from-pulling-in-subtitles/826314/14

The workaround provided does work and would then allow this PR to work as expected.