Add simple script to handle transcoding of video files

Currently, takes a file that has:

* track 1: h.264 video
* track 2: AC3 audio
* track 3: subtitles

and converts to:

* track 1: h.264 copied
* track 2: AAC audio converted
* track 3: AC3 audio copied
* track 4: subtitles converted to `mov_text`

TODO: Set this up with help text, and switches to take:

* h.264 w/ AAC
* h.264 w/ DTS
* h.264 w/o subtitles for all audio situations