Add support for subtitles

[mstrelan]

It would be good if this script would allow converting subtitles also. Example ffmpeg usage:

C:\ffmpeg.exe -i "D:\input.mkv" -vcodec copy -acodec copy -scodec mov_text -map 0 D:\output.mp4

MP4 output requires the mov_text codec, but other containers could use copy instead.

The option -map 0 also copies ALL streams instead of just the first of each. Perhaps you want to expand upon that as well as per http://ffmpeg.org/ffmpeg.html#Subtitle-options_003a so users can copy all subtitle streams.

[cytec]

maybe this helps #33

[senko]

Since #33 is now merged, I believe all of this now works, right @cytec?

[cytec]

yes, map is implemented as well as subtitle codecs