This only supports TTML output; meaning the user can convert WebVTT
into TTML, but not the other way around. This will be useful for
DVB-sub subtitles that would be better supported within TTML.
This only adds text-based output; a follow-up will add MP4 support.
Change-Id: I0944b7df95d7765e55f203fc5e9a644f5c455dd8
Jacob Trimble