title: Firebase custom OAuth authentication
body: <p>FireBase appears to support OAuth-based authentication from Facebook, Twitter, Google, GitHub. What about a different OAuth provider? It would be great if there were a generic "OAuth" option where in addition to specifying an API Key and Secret you specified whatever other information was needed.</p>
  
  <p>Is it possible to use FireBase's existing OAuth-based authentication modules without rolling my own custom OAuth authentication module for FireBase, by possibly forking from one of the "built-in" OAuth mechanisms? Which one is the most generic, if so?</p>