Fire additional event for incremental auth scenarios

This would allow custom sign-in elements to make use of the full `google-signin` functionality.

In [my sample for custom sign-in](https://github.com/Scarygami/google-signin-samples/blob/master/elements/custom-google-signin.html) I can only have sign-in and sign-out buttons since I won't be notified about additional necessary permissions with the current implementation.