- 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

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

MS Office XML rsid is a "unique identifier used to track the editing session
when the physical character representing this section mark was last formatted."

See the following links for details:
- https://msdn.microsoft.com/en-us/library/office/documentformat.openxml.wordprocessing.previoussectionproperties.rsidrpr.aspx
- https://blogs.msdn.microsoft.com/brian_jones/2006/12/11/whats-up-with-all-those-rsids/.

In XML, the order of the attributes shouldn't be meaningful,
however, MS Office sorts attributes for a given XML tag
differently than LibreOffice.

Closes #60

Note: this changeset also ensures that clean.cleaned.docx is removed
up after the pytest is over.