Extend subtitle cutter to work with .ass subtitles

[LolicoreExpress]

I'm not sure if it's normal or not (ass format not yet implemented for example) But I tried making some ASS subs and they plain didnt work. When launching the encode process, each webm juste "stopped" at half encode completion.

[dfaker]

The .ass subtitles can be used with the standard subtitle filter, but the subtitle cutting logic is .srt only so it'll have to be .srt format for now.