- 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

Use color palette an size according to
https://developer.gnome.org/hig/stable/icon-design.html.en

Relates 0a2a398c

We didn't take the whitelist into account while
removing dangling files from [Content_types].xml

- Replace some class members by instance members
- Don't thread the cleaning process anymore for now