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)
With Python 3, the titlebar wouldn't get updated for a long time (due to
stderr buffering), and when it did, the title would be shown as b'...'
representation.