generic subtitle download.
The idea is that all subtitle downloaders must descend from SubtitlesIE
and implement only three basic methods to achieve the complete subtitle
download functionality. This will allow to reduce the code in YoutubeIE
once it is rewritten.
In order to be more consistent with different subtitle formats.
From:
* --write-srt to --write-sub
* --only-srt to --only-sub
* --all-srt to --all-subs
* --srt-lang to --sub-lang'
Refactored also all the mentions of srt for sub in all the source code.
Improved option '--srt-lang'
- it shows the argument in case of missing subtitles
- added language suffix for non-english languages (e.g. video.it.srt)
Ismael Mejia
