title: Unable to change font color of Select2 dropdown with custom attributes
body: <p>Currently trying to use Select2 in an old app. I can successfully activate the old dropdown and turn it into a Select2 dropdown. All works fine. The dropdown is filled up and related functions are still working. However, I'm trying to do something a bit more advanced. </p>
  
  <p>In the old dropdown, every option has a custom attribute. </p>
  
  <pre><code>&lt;option onlyslave="True" ...&gt; ...&lt;/option&gt;
  </code></pre>
  
  <p>Now, if the onlyslave property is true, I want the text color to be red. If it's false, it should remain black. I already know how to set the text to red, using this: </p>
  
  <pre><code>.select2-results { 
    color: red;
  }
  </code></pre>
  
  <p>That obviously turns every result in the Select2 dropdown red. I thought adding an attribute [onlyslave="true"] would solve it, but it does not. The following snippet doesn't turn anything red, despite the fact that the 'old' dropdown, behind the screen, has options with False &amp; Truth. Capital 'Truth' or 'truth' make no difference here.</p>
  
  <pre><code>.select2-results[onlyslave="true"] { 
    color: red;
  }
  </code></pre>
  
  <p>Anybody know how I can accomplish this? I find the Select2 documentation a bit lackluster. I'm currently using Select2 4.0.5, but might be able to downgrade. </p>