Support SSA/ASS styling

Add support for SSA/ASS styling. Currently there is only "minimal" support for ASS subtitles since styling was removed from the initial PR. (See https://github.com/google/ExoPlayer/pull/2281)

It would be great to have full support of the spec. Many subtitles use these styling features to work directly with the video (e.g. to substitute text in a video). SSA/ASS styling is also very common in certain genres of media like Cartoons, Anime or Karaoke.