Add some lightweight typing

[jvoisin]

This is a first quick pass over the codebase. If having better typing is something desirable, I'll do another more comprehensive one.

Having typing makes it easier to understand what's going on in the code, eg. "this function called colorize_result it taking either a str or None, and is returning either a str or None, so odds are that it's handling error conditions properly." It also makes the life of IDE/static analyzers/… easier.