title: Does angular ui select (bootstrap version) support optgroup?
body: <p>Does anyone know if Angular-Ui-Select Bootstrap version support an <strong><code>optgroup</code></strong>?</p>
  
  <p>Can't seem to find any documentation for that on <a href="https://github.com/angular-ui/ui-select" rel="nofollow">https://github.com/angular-ui/ui-select</a>?</p>
  
  <p>Here is their example:</p>
  
  <pre><code>plnkr.co/edit/QCwSM75ilH2Vh6D9aMA4?p=preview
  </code></pre>
  
  <p>How to add an <code>optgroup</code>?</p>
  
  <p>In this example, lets say, group persons by countries.</p>