Allow downloading closed captions track as subtitles

[KamasamaK]

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Twitch natively supports Closed Captions, so it would be great if this tool could download those from a VOD as a subtitle track. Unfortunately, they don't seem to be currently supported in Clips on Twitch.

[ScrubN]

I have made some progress on this.

VOD captions are stored inside the video part files, which is convenient. However there is no way to tell if a given VOD contains any subtitle data without scanning every single video part, which can take quite a while. My experimenting has shown ~1.6s per video part (10s) at best when tested on https://www.twitch.tv/videos/1866410218.

I think captions scanning will need to be opt-in because of this.