This should ensure that if we decide to implement
some threading in the cli, a faulty file
won't break everything.

- More tests
- More documentation
- Minor code cleanup

This should improve epub compatibility,
along with other formats as a side-effect

This should make easier to get more metadata from
archive-based file formats.

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.

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

This should close #74