Add support for m4v files

This allows transcription of "m4v" files when placed in the incoming_mp4 bind mount directory. Previously, only "mp4" files were supported by this tool. Caption Whale will now transcribe m4v files in addition to mp4 files.

This was tested successfuly on files from Virginia Tech's repository. These files do not have DRM features of M4V enabled. It's possible (and quite likely) that Caption Whale will fail when attempting to transcribe copy-protected "m4v" files.