public function registerFormat($format, $class)
    {
        if ( ! class_exists($class)) {
            throw new \InvalidArgumentException("Responder formatter class {$class} not found.");
        }
        if ( ! is_a($class, 'Nathanmac\Utilities\Responder\Formats\FormatInterface', true)) {
            throw new \InvalidArgumentException('Responder formatters must implement the Nathanmac\Utilities\Responder\Formats\FormatInterface interface.');
        }

        $this->supported_formats[$format] = $class;

        return $this;
    }