title: Allow login using username or email in Laravel 5.4
body: <p>Now I've followed the Laravel documentation on how to allow usernames during authentication, but it takes away the ability to use the email. I want to allow users to use their username or email to login. How do I go about this?</p>
  
  <p>I've added this code to the LoginController as per Laravel's Documentation and it only allows username for login. I want it to accept username or email for login.</p>
  
  <pre><code>public function username () {
      return 'username';
  }
  </code></pre>