Warn existing users on sign in

Thanks to @damonmorgan for contributing this new feature.

This releases adds a new feature, hooking the pwned password check into the sign in path of an existing devise user. In the readme is an example of how to use this to push a flash notification after sign in to warn users if they are using a password that previously appeared in a data breach.

You could also use this hook to redirect users to a change password page or similar.

This change is backwards compatible, so you can upgrade without changing existing behaviour.