Add event callback prop to Auth UI component

It would be handy if the Auth component received an event callback prop so that developers could trigger actions when the internal state changes.

The usage in React might look something like this:


My specific use case: When a user signs up I would like to hide the Auth component and display a custom component view in its place.

I would be happy to submit a PR for this feature but I would like to receive feedback on the feature idea before starting on the implementation.