Bind to event other than native scroll

[sjwilliams]

This change gives us the option of monitoring an event other than native scroll to check for updates.

This is useful in cases, like at The New York Times, where we already have a view that is monitoring scroll events and dispatching its own, throttled version of scroll. I'd rather subscribe to these site-wide events than attach an additional handler to the scroll. Here's the exact implementation I'm developing against:

  $container.scrollStory({
    content: $items,
    scrollEvent: function(cb){
      PageManager.on('nyt:page-scroll', cb);
    }
  }).data('plugin_scrollStory');

The new scrollEvent option can be a function that calls the update method as a callback, like the example above, or a string that specifies an event on the window to listen for. The default is the string 'scroll', which gets translated to our current technique, monitoring $(window).scroll() with a throttle determined by scrollSensitivity and throttleType.

If another string is passed in, we monitor the window for that event. Something like this:

// some code dispatching throttled events
$window.trigger('nytg-scroll');

 $container.scrollStory({
   scrollEvent: 'nytg-scroll'
 });