Reverse proxy routes

[stuartpb]

I'm thinking this will be handled with a new, special, not-necessarily-supported route type, like list in #6. Say, instead of /foo/bar 200 localfile.html, you'd have /foo/bar proxy 200 http://remote.example.com/filelocation.html, with a /foo/bar 301 http://remote.example.com/filelocation.html right beneath it for implementations that don't recognize proxy.