- More tests
- More documentation
- Minor code cleanup

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

- 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

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.

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