Feature Request: Type Hints

[AndrewMHenry]

Hello,

Would it be feasible to add type hints for all or part of the code? For example, if I call the .render() method on a document, mypy infers the return type as Any and complains when I try to use it as a string.

Also, thanks for all your work on this library! Using it makes code so much cleaner and more readable than ad-hoc implementations.