title: adding an event listener to impress.js
body: <p>I am using <code>impress.js</code> library to make a presentation. I would like to attach a listener for a <code>div</code> where if the slide has classes <code>present active</code>. Then, I would like to have a counter to be displayed on the slide. I tried to use <code>elem.addEventListener('change',function(){counter()},false)</code>, but it didn't work.</p>
  
  <p>I would like to know if there is a way to attach an event to <code>impress:next</code> function. </p>
  
  <p>Sincerely,
  Abdul </p>