feature(audioPassthrough): adding in basic audio passthrough functionality

Attempting to allow audio passthrough for defined codecs, per the existing ticket and my own needs. Usage is through --audio-passthrough=CODEC,CODEC2, and --use-audio-passthrough to simply passthrough all audio tracks. One additional feature crept in as well, the ability to preserve the number of audio channels in the source via the option --audio-channels=KEEP.
feature(audioPassthrough): change keyword for preserving source audio channels to SOURCE
feature(audioPassthrough): clean up in opts passing, incorporate fix

Slight cleanup and bringing in the small fix I submitted as another PR since maintaining the two branches is a pain
better variable name
feature(overwrite): die if output file exists, add overwrite flag

I noticed that pre-existing mp4 containers wer sometimes having tracks added if they already existed. Adding failure with overwrite flag that explicitly removes the existing output file first to deal with it
feature(mp4boxTmp): adding a flag to set a mp4box tmp folder

Useful for me when running mkvtomp4 from applescipt which has some sort of issue with the tmp folder. Seemingly related to this http://sourceforge.net/p/gpac/bugs/242/
feature(mp4boxTmp): adding help text
feature(subtitles): Adding basic support for subtitle conversion

Only works with mp4box, has flags --subtitle-font and --subtitle-font-size for customizing subtitle appearance
Merge branch 'overwrite'
Merge branch 'mp4box-tmp'
resolving merge conflicts
resolve merge conflicts