Callback function support for login redirect route

I've added support for using a callback instead of a fixed string for the login_redirect_route. This will allow the coder to dynamically construct the redirect route based on the user object.

The most common use case would be of role-based redirection.

Usage example:
'login_redirect_route' => function(\ZfcUser\Entity\UserInterface $user) {
    // Any custom logic
    return 'my-custom-process-route';
}