Save subtitles using the same code for all the options
Use a dictionary for storing the subtitles

The errors while getting the subtitles are reported as warnings, if no subtitles are found return and empty dict.
YoutubeIE: use the same function for getting the subtitles for the "--write-sub" and "--all-sub" options
YoutubeIE: don't crash when trying to get automatic captions if the videos has standard subtitles.
Don't try to write the subtitles if it's None
Merge remote-tracking branch 'upstream/master'
Update generator

Signed-off-by: Emilien Kenler <hello@emilienkenler.com>
Update algo - see #1254

Signed-off-by: Emilien Kenler <hello@emilienkenler.com>
Merge branch 'master' into py-generator
Merge remote-tracking branch 'upstream/master'
Add YouTube DASH formats to YouTubeIE
fix failed tests
fix code style
address review comment
fix hls test
Merge pull request #1266 from MiLk/py-generator

Update the youtube algorithm generator
Merge pull request #1279 from xanadu/master

Add YouTube DASH formats to YouTubeIE
[youtube] fix the order of DASH formats
[youtube] update algo for length 82 (fixes #1296)
[subtitles] separated subtitle options in their own group