title: Enable angular-ui tooltip on custom events
body: <p>I am trying to use angular-ui's tooltip functionality to show my user that a particular field is invalid but it seems that the tooltip can be shown only on some pre-defined triggers. Is there any way by which I can trigger the tooltip except those triggers?</p>
  
  <p>For example:</p>
  
  <pre><code>&lt;input
      type="text"
      tooltip="Invalid name!"
      tooltip-position="right"
      tooltip-trigger="myForm.username.$invalid"&gt;
  </code></pre>