title: Select2 v4.0 make optgroups selectable
body: <p>I'm using the latest version of select2 (4.0.0) and I can't find the option to make optgroups selectable.</p>
  
  <p>An optgroup is used to group different options of the dropdown, as shown in their basic examples:
  <img src="https://i.stack.imgur.com/rubHZ.png" alt="Example for an optgroup"></p>
  
  <p>I need this optgoup to be selectable too! It was possible in 3.5.1 but it isn't the default setting in 4.0.0 anymore.</p>
  
  <p>My Code Looks like this:</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false" data-console="false" data-babel="false">
  <div class="snippet-code">
  <pre class="snippet-code-js lang-js prettyprint-override"><code>$(document).ready(function() {
     $('#countrySelect').select2({
       data: [{
         text: "group",
         "id": 1,
         children: [{
           "text": "Test 2",
           "id": "2"
         }, {
           "text": "Test 3",
           "id": "3",
           "selected": true
         }]
       }]
     });
   });</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"&gt;&lt;/script&gt;
  &lt;script src="https://select2.github.io/dist/js/select2.full.js"&gt;&lt;/script&gt;
  &lt;link href="https://select2.github.io/dist/css/select2.min.css" rel="stylesheet" /&gt;
  
  
  &lt;select id="countrySelect" style="width: 380px;" placeholder="Select regions..."&gt;&lt;/select&gt;</code></pre>
  </div>
  </div>
  </p>