title: How to use format() on a moment.js duration?
body: <p>Is there any way I can use the <a href="http://momentjs.com/" rel="noreferrer">moment.js</a> <code>format</code> method on duration objects?  I can't find it anywhere in the docs and it doesn't seen to be an attribute on duration objects.</p>
  <p>I'd like to be able to do something like:</p>
  <pre><code>var diff = moment(end).unix() - moment(start).unix();
  moment.duration(diff).format('hh:mm:ss')
  </code></pre>
  <p>Also, if there are any other libraries which can easily accommodate this sort of functionality, I'd be interested in recommendations.</p>
  <p>Thanks!</p>