Kudos to Sherry Taylor for reporting this issue 

- This commit introduces optional dependencies (namely ffmpeg):
  mat2 will spit a warning when trying to process an .avi file
  if ffmpeg isn't installed.
- Since metadata are obtained via exiftool, this commit
  also refactors a bit our exfitool wrapper.

Fixes issue #61

Since FLAC files can contain covers, it makes sense
to parse their metadata

This should close #74