Support proxy basic authentication

[juggernaut]

Support proxy auth via the Proxy-Authorization header.

• The username and password must be configurable in the yaml
• The password in the configuration must not be accepted in plain text -- it should be hashed using bcrypt
• The password in the request must be checked against the configured password using constant time comparison
• If the request does not contain a Proxy-Authorization header, a 407 Proxy Authentication Required response must be returned along with the appropriate header as specified in https://tools.ietf.org/html/rfc2616#section-14.33