koa-grant: How to get the user info from session
I want to find a way to get the user info from the session after login. 

For example:

``` js
// suppose user have login
router.get('/demo', function *(next) {
  if (!this.session.grant) {
    this.redirect('/login');
  } else {
   // get the user info from DB which I saved after user login successfully,
   //    eg, saved  in `/hanlde_provider_callback
    user = yield getUserInfo(this.session.grant.userId);
  }
}
```

Seems I can't add a new property to session in the `/handle_provider_callback`, so I don't how to connect  the special session with the db item (I want to store users in my db). Do you have any idea?

Thank you.
