title: how to get option group of selected item on .chosen().change()
body: <p>For a simple option grouping, say:</p>
  
  <pre><code>   &lt;optgroup label="fruit"&gt;
       &lt;option value="1"&gt; apples &lt;/option&gt;
       &lt;option value="2"&gt; pears &lt;/option&gt;
    &lt;/optgroup&gt;
    &lt;optgroup label="veg"&gt;
       &lt;option value="3"&gt; neeps &lt;/option&gt;
       &lt;option value="4"&gt; tatties &lt;/option&gt;
    &lt;/optgroup&gt;
  </code></pre>
  
  <p>I'm able to get grab the selected option's id...  Using:</p>
  
  <pre><code>    $('#my-chzn').chosen().change(
          function(evt) {
             var id = $(this).val();
             // ...or 
             var id_ = $(evt.target).val();
          }
      );
  </code></pre>
  
  <p>But is it possible to grab the <code>&lt;optgroup&gt;</code> label for a selected option? 
  ie is there a handle/selector to grab the value 'fruit' when the selected option is 'pears'? </p>
  
  <p>Many thanks for any help anyone's able to offer....  </p>