Configure all the things!

This release adds 2 features:

1. didScroll{Up,Down,Left,Right} hooks

   You can now subscribe to didScroll{Up,Down,Left,Right} hooks that are
   emitted by the Mixin when the element enters the viewport. This is primarily
   for the purposes of replacing the waypoints library in ember-waypoints to
   use an Ember implementation instead.

   As with any other hook in Ember, you can handle it with:

   export default Ember.Component.extend(InViewportMixin, {
     didScrollUp(direction) {
       console.log(direction); // 'up'
     },
   
     didScrollDown(direction) {
       console.log(direction); // 'down'
     },
   
     didScrollLeft(direction) {
       console.log(direction); // 'left'
     },
   
     didScrollRight(direction) {
       console.log(direction); // 'right'
     }
   });

   There is also a new viewportScrollSensitivity option that affects the above
   behavior.

2. Global options

   You can set application wide defaults for ember-in-viewport in your app
   (they are still manually overridable inside of a Component). To set new
   defaults, just add a config object to config/environment.js, like so:

   module.exports = function(environment) {
     var ENV = {
       // ...
       viewportConfig: {
         viewportSpy               : false,
         viewportUseRAF            : true,
         viewportScrollSensitivity : 1,
         viewportRefreshRate       : 100,
         viewportListeners         : [],
         viewportTolerance: {
           top    : 0,
           left   : 0,
           bottom : 0,
           right  : 0
         }
       }
     };
   };

Changelog

#13 Application wide defaults by @poteto
#12 Guard against removed element by @LevelbossMike
#11 waypoint hooks first draft by @poteto