public function emailOutput($address)
    {
        if (is_null($this->_output)
            || $this->_output == $this->getDefaultOutput()
        ) {
            throw new InvalidCallException('Must direct output to file in order to email results.');
        }
        $address = is_array($address) ? $address : func_get_args();

        return $this->then(function (Application $app) use ($address) {
            $this->sendEmail($app->mailer, $address);
        });
    }