title: OmniAuth dynamic callback url to authenticate particular objects instead of current_user
body: <p>Say I have the models User and Project. Users and projects are HABTM-associated. My setup is actually a bit more complicated than this, but I think for the purposes of my question this will do.</p>
  
  <p>Now, I want to use omniauth to authenticate a particular project with Twitter, Facebook, what have you. I've figured out how to define my omniauth path_prefix, but I don't know how I could pass in a variable like so: <code>config.path_prefix = 'projects/:project_id/auth'</code>, much less make a custom callback url like project/:project_id/auth/twitter/callback.</p>