Add middleware Raxx.BasicAuth

[CrowdHailer]

This should be started in a new repo and linked from the extensions section of this repo.
The hex project should be raxx_basic_auth.

The functionality needs only to match what is in this repo. https://github.com/CultivateHQ/basic_auth
It is important that this middleware can handle streamed responses

[CrowdHailer]

Another basic auth implementation https://github.com/bitgamma/plug_auth/blob/master/lib/plug_auth/authentication/basic.ex