This should close #28

Those attacks are DoS-only.

- 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

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