We're using methods that aren't available in Poppler
below 0.46, so we're checking for this upon import.

This commit is based on ideas from @LogicalDash 

- xml files are no longer considered harmless
- Factorization of the `remove_all` method for office files
- Explicit whitelist are used
- Blacklist are used to skip files completely
  - Non-blacklisted files are _still cleaned_
  - Unsupported files are still triggering an error

Fedora defines the 'text/xml' mime type for xml files. Adds this mime
type to the harmless parser.

Fixes #36.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

The FLAC parser looks for the 'audio/flac' mime type, but Fedora
defines 'audio/x-flac' in /etc/mime.types for FLAC files. Add this mime
type to the audio parser.

Fixes #36.

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

The dynamic import should now work when MAT2 is
installed system-wide, either via the distribution's
packaging system, or via pip.

- the `src` folder is now `libmat2`
- the `main.py` script is now `mat2.py`