title: DJANGO: Add "select all" choice option to ModelMultipleChoiceField?
body: <p>Is there a way to add additional choices to a ModelMultipleChoiceField? Specifically, I want to add a "Select All" option instead of an End-User having to select all of the choices individually.</p>
  
  <p>Current Model Form:</p>
  
  <pre><code>class QueryForm(forms.Form):
      book = forms.ModelMultipleChoiceField(queryset=Book.objects.all())
  </code></pre>
  
  <p>I want something like this:</p>
  
  <pre><code>class QueryForm(forms.Form):
      book = forms.ModelMultipleChoiceField(choices=(('Select All', 'Select All'),(Book.objects.all()))
  </code></pre>
  
  <p>How can I add the Select All option to Model Multiple Choice Field?</p>