title: passport-facebook: how to dynamically set callbackURL?
body: <p>I'm following this example to setup a FacebookStrategy - <a href="https://github.com/jaredhanson/passport-facebook/blob/master/examples/login/app.js" rel="noreferrer">https://github.com/jaredhanson/passport-facebook/blob/master/examples/login/app.js</a></p>
  
  <p>I want to set the callbackURL dynamically,
  ie: the user access /posts/34 so the callback will be /posts/34/callback</p>
  
  <p>how can I achieve something like this ?</p>