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 pull request #937 from jaimeMF/subtitles_rework

Subtitles rework