Add with_basic_auth() function

[chriscrabtree]

First, I really like the design of this: feels idiomatic, extensible, and explicit.

Anyway, I added this with_basic_auth() in my own client code at first, but thought it would be more naturally at home as part of this package. Happy to submit a PR if you like (and if you don't mind dealing with a first-time open-source contributor).

This is the shape of it:
def with_basic_auth(request = %HttpRequest{}, username, password) when is_binary(username) and is_binary(password) do...

Thanks!

[matthewoden]

Hey! I just saw this - I think my notification settings are borked. This would be a great addition.

I'm a little swamped, so I'd be delighted to take a PR.