title: count the number of days in a given date range in months, days using momentjs
body: <p>How do i get the exact number of days from a given date range? 
  ex. given the following:</p>
  
  <pre><code>today = moment("2016-06-29")
  due = moment("2016-05-13")
  
  days_left = today.diff(due, 'days'); // returns 47
  </code></pre>
  
  <p>but i want it to display 1 month and 16 days ?</p>