title: Fullcalendar.io dynamic data loading for currently visible period
body: <p>I have a large event dataset so I'd like to find a way to load data for the period (start &amp; end date range) currently visible calendar.</p>
  
  <p>I tried the following hoping that "prev" somehow accepts callback, but it doesn't work. </p>
  
  <pre><code>$('#calendar').fullCalendar({
   prev: function() {
      console.log('prev button pressed. data for the current date range will be fetched.');
    }
  })
  </code></pre>