Mandatory base URL support in config

[joshtriplett]

I'd like to set a flag in Config that forces the resulting Client to only use URIs relative to the specified base URI, and never support absolute URIs.

This would add security when providing authentication: the config could add_header a header containing authentication information, and also set a mandatory base URI to ensure that the Client can't accidentally pass that authentication information to any other domain.

[joshtriplett]

#343