- 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

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/.

Yoann Lamouroux authored 6 years ago and jvoisin committed 6 years ago

`/usr/bin/python3` -> `/usr/bin/env python3`

It's always better to trust the environment defined path to bin/python, as
virtualenv become the way to go.

Closes #60

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

This allows the user to make use of parser.unknown_member_policy for
archive formats.

At the suggestion of @jvoisin, it also prints a scary warning if the
user explicitly chooses 'keep'.

This should close #28

Example use:

```
$ mat2 -c
Dependencies required for MAT2 0.1.3:
- Cairo: yes
- Exiftool: yes
- GdkPixbuf from PyGobject: yes
- Mutagen: yes
- Poppler from PyGobject: yes
- PyGobject: yes
```

This should close #35