Add type hinting

[sandalwoodbox]

Currently, mypy checking of my python code fails with:

 error: Skipping analyzing 'fluent.runtime': found module but no type hints or library stubs

I'll just ignore the error for now but it would be nice to have type hint support. It looks like this is blocked by #162.

[Pike]

Also a port of projectfluent/fluent.js#524, I'd say.