Add an option to download automatic captions from Youtube

[jaimeMF]

Some users may not want to download the automatic captions if the subtitles are not found.
The question is if the default behaviour should be to download them or not.
Suggested name for the option: --write-automatic-sub or --write-auto-sub (by iemejia) --write-automatic-srt.

[iemejia]

Hi, I refactored and wrote a good part of the support for subtitles some time ago. And I tried to unify the use of 'sub' in the options, since other formats than srt are supported (or could eventually) as you can notice the old '--write-srt' is only kept for compatibility, so my suggestion would be maybe '--write-automatic-sub' or '--write-auto-sub' that it's shorter.

[iemejia]

btw, nice proposal.

[jaimeMF]

Thanks, your name is better, I haven't noticed --write-srt was deprecated