Add type hinting and a `py.typed` marker file

[j00bar]

It would be awesome to be able to use this library in projects that are type checked by mypy without having to add it to the ignores.

To do that, for the installable code, you'd need to type hint the arguments and return for each function and then touch a py.typed file in the module to alert mypy it's safe to use the annotations on the code itself.

[jrobichaud]

I have a stash on my computer where I started to work on the type definitions.

I'll complete it now that someone asks for it.