Make subtitles optional

[nschlia]

Currently subtitles are always transcoded if possible. This should be made optional, I have found at least one video source that does not play in hls.js when subtitles are present in the output files. hls.js does not support embedded subtitles, thus it is useless to provide these streams for that audience.
An option --no_subitles should be added to turn them off.