--force-subtitle and -S did not respect each other but now it is
working as it should. If --force-subtitle is used with -S it downloads
only the subtitles, and if --force-subtitle is used alone it overwrites
the existing subtitles and continues with the audio/video stream.
The option gets the correct ISO639-3 language code using my own simple
language detection api, and merges with correct metadata using ffmpeg.
If ffmpeg is not installed an error message is raised since avconv
doesn't seem keep up with ffmpeg.
I also did a little cleanup in postprocess().
None of these were any real problems, but easier to spot real issues if pylint
is a bit quieter. Apart from the pylint overrides being sprinkled over the code
base, this commit also fixes occurences of the following issues:
- logging-not-lazy
- logging-format-interpolation
- unused-import
- unused-variable
