Pull credentials from HTTP Basic Auth

[dustinsmith1024]

I just implemented passport-windowsauth and everything is going well. We are using HTTP Basic Auth to send the user/pass. So we pull the user/pass from the Authorization header first and push them onto the request.body.username and .password fields.

It's a bit awkward to populate the request.body manually. It'd be nice if we could remove this first piece of middleware and just have it all integrated here. Would you consider a PR for integrating Basic auth?

If not, we could make the user/pass assignment into a function that can be configured.
https://github.com/auth0/passport-windowsauth/blob/master/lib/strategy.js#L121

[jfromaniello]

Yes, it should be an option like getUsernameFromHeader for integrated auth.. Maybe getCredentials and that takes the credentials from body by default..?