title: Add function to laravel Notification
body: <p>I have next Notification class:</p>
  
  <pre><code>class WelcomeNotification extends Notification
  {
      use Queueable;
  
      public function __construct()
      {
          //
      }
  
      public function via($notifiable)
      {
          return ['database'];
      }
  
      public function toDatabase($notifiable)
      {
          return [
              //
          ];
      }
  }
  </code></pre>
  
  <p>And I want add some function to this. For example:</p>
  
  <pre><code>public function myFunction()
  {
      return 'something';
  }
  </code></pre>
  
  <p>But $user->notifications->first()->myFunction return nothing</p>