title: Tri-state md-checkbox with angular materials
body: <p>I'm working with <strong>Angular material 1.0.5</strong> and the <strong><a href="https://material.angularjs.org/1.0.5/api/directive/mdCheckbox" rel="noreferrer">md-checkbox</a></strong> directive.
  I was wondering if anyone knows how to make this into a tri-state checkbox. </p>
  
  <p>The three states (and the associated variable values for my situation) are:</p>
  
  <ul>
  <li>Checked   (true)</li>
  <li>Unchecked (false)</li>
  <li>Indeterminate (null)</li>
  </ul>
  
  <p>For the version of Angular Material specified (1.0.5), when the checkbox is disabled, it
  shows the indeterminate state as a checkbox with a question mark in it.</p>
  
  <p>However, when it is not disabled, it defaults back to a two state checkbox.</p>
  
  <p>So far my failed attempts have been to wrapping the directive in another directive and trying to take over control of the md-checkbox. </p>
  
  <p>Does anyone have any pointers in this situation?</p>
  
  <p>Thanks.</p>