title: django-mptt count items in add_related_count?
body: <p>How to add <code>filter(is_public=True)</code> to Question?</p>
  <pre><code>Category.objects.add_related_count(
    Category.objects.filter(is_public=True), 
    Question,
    'category',
    'question_counts',
    cumulative=True)
  </code></pre>