Allow options per hasher class for Fallback.

This will be most useful for people upgrading and using different hash algos as the default sha1 or need to pass options for specific hashers in general.
Actually, the other side also only uses a single is_string() check: https://github.com/cakephp/cakephp/blob/3.0/src/Auth/PasswordHasherFactory.php#L36
Maybe we should use that instead?