title: Get custom data-attribute in select2 with <select>
body: <p>Let's assume you have the following HTML5</p>
  
  <pre><code>&lt;select id="example"&gt;
      &lt;option value="AA" data-id="143"&gt;AA&lt;/option&gt;
      &lt;option value="BB" data-id="344"&gt;BB&lt;/option&gt;
  &lt;/select&gt;
  
  $("#example").select2();
  </code></pre>
  
  <p>How do I get the data-id from the selected option ?</p>