Add support for VBR MP3

This adds support for VBR MP3 which produces much smaller files. The support in `mp4` is still experimental so I haven't added it.
FFmpeg docs: https://trac.ffmpeg.org/wiki/Encode/MP3

Granted the lines `296-302` aren't the sexiest but I didn't see any better way doing it quickly. Happy to change that if there's another place in the code I haven't seen.

Also, I've added a line about installing `ffmpeg` on OSX along with the ogg codecs. An error I ran into immediately.

Thanks for making such a useful module!