Add class to enable loading of out-of-band subtitle files.
Add missing param documentation.
Strip trailing newline from WebVTT subtitles.
Improve subtitle handling.

- Move parsing onto a background thread. This is analogous
  to how frame decoding is pushed to MediaCodec, and should
  prevent possible jank when new subtitle samples are parsed.
  This is more important for out-of-band subtitles, which can
  take a second or two to parse fully.

- Add Useful DataSpec method.