always push 'next' to the route handler fn args

[soygul]

This will make it possible to yield downwards even inside a handled route, like:

  app.use(route.get('/test/:param', function *(param, next) {
    // ...
    yield next;
  }));

[tj]

LGTM but a test would be great

[soygul]

Didn't update the example in the readme not to spoil the simplicity of the example