Implement scroll tracking feature

[jeremydw]

• Some sample code below, where ratios is a list of breakpoint percentages (of the page height) to track.
• A similar utility could be written using airkit's inview module, to track events when specific elements enter view.

var ratios = [0.3, 0.6, 0.9];
var func = function(category, action, label) {
    trackEvent(category, action, label);
};
window.addEventListener('scroll', function() {
  var scrollRatio = window.scrollY /
      (document.body.offsetHeight - window.innerHeight);
  for (var i = 0; i < ratios.length; i++) {
    if (scrollRatio > ratios[i]) {
      var percent = ratios[i] * 100;
      func('EVENT NAME', 'EVENT ACTION', percent + '%');
      ratios[i] = 200; // Some number out of bounds.
    }
  }
});