Adding type hints

[k3-cat]

Mutagen is a brilliant tool for view and edit media metadata, however, I've found that it is currently lacking of type hints.

Type hints can help IDEs or linters do many things such as static syntex check, autocomplete and so on. Every new version of Python are adding new function about type hints now. The best part is, Python 3.6 already support some basic hinting grammar.

If you don't mind, I can help with adding type hints for those commonly used classes.