Allow passthrough

This allows someone to wrap a layout with express. Not sure if doing this the best way, but see implementation [in this repo](https://github.com/natew/reactor), specifically the app/server.js. Something like this:

    .use(ReactMiddleWare(App))
    .use(function(req, res, next) {
      var response = fs.readFileSync(layout).replace('</head>', '</head>' + res.body);
      res.send(response);
    })

Would be interested in learning a better way to allow this.