Reimplement subtitle support

[chrisbenincasa]

Since we've removed the Plex transcoder, we need to bring back subtitle support with an 'in house' implementation. Of course, this is quite complicated, so we'll probably break it up into chunks:

1. Support for external subtitles first. These are simplest because we can just pull the file from Plex and feed it into the stream.
2. Support for extraction of embedded subtitles -- burning embedded subtitles into the output stream works well for local FS files (perhaps this can be an option with the 'direct' stream path), but when the stream input is coming from over the network, ffmpeg attempts to extract the subtitles out of the stream before playback starts, which is obviously a non-starter (it delays the stream)
3. Exposing these options and capabilities in the UI to some degree (e.g. "enable burn-in", background task configuration)