add support for _ default key

[ethanabrooks]

In reference to #34

[jspahrsummers]

Thanks for the pull request! Could you please add a test demonstrating how to use this?

[ethanabrooks]

done!

[ethanabrooks]

Hmmm I took a look at the typechecker, but it wasn't exactly clear how to add _ to the list of valid keyword arguments. Could you lend some guidance on that front?

[jspahrsummers]

Thanks for the test! It's been a while since I've hacked on the codebase myself, to be honest, but at a high level it would involve telling the typechecker about your new argument to match.

The tricky part that I foresee here is that the type of _ is quite open-ended (probably something like Callable[..., _MatchResult]). I'm not sure what mypy's API for that looks like, but it would look broadly similar to how we instantiate the other mypy.types.CallableTypes.

Hope that helps.