title: Selectize.js: add category label to the selection
body: <p><a href="https://github.com/brianreavis/selectize.js" rel="nofollow noreferrer">selectize.js</a></p>
  
  <p>It is possible to add a non selectable category (label) to the input selection? </p>
  
  <p><a href="https://i.stack.imgur.com/etbyC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/etbyC.png" alt="enter image description here"></a></p>
  
  <p>The problem is, that I have a big list of inputs and it would be great to add categories (labels) to the selection. Like this:</p>
  
  <pre><code>Category 1
  ---Item1
  ---Item2
  ---Item3
  ---Item4
  Category 2
  ---Item5
  ---Item6
  ---Item7
  ---Item8
  ...etc
  </code></pre>