Allow a simple way to specify an origin host

When dealing with large farms of hosts, or non-production instances, it'd be awesome to send a request for http://foo.com/ to a different host than foo.com (like test.foo.com) by optionally showing an "origin" text field.

This is exactly the same as requesting http://test.foo.com/ with a "Host: foo.com" header.  The UI is just different and much clearer for most users.
