Added support for additional vtt subtitle format (WebVTT) in youtube-dl.
Added missing write-auto-sub option in README file
Fixed typo in subtitle format option (from: sbt => sbv)
Merge pull request #936 from iemejia/master

Added option for vtt WebVTT subtitle format for Youtube
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