Hook to override default toggle visibility in _isVisibleDidChange

At the moment there is no way apart from overriding the whole of _isVisibleDidChange to supply your own custom animations for when the view's isVisible property changes:
```
  _isVisibleDidChange: Ember.observer(function() {
    var $el = this.$();
    if (!$el) { return; }

    var isVisible = get(this, 'isVisible');

    $el.toggle(isVisible);

    if (this._isAncestorHidden()) { return; }

    if (isVisible) {
      this._notifyBecameVisible();
    } else {
      this._notifyBecameHidden();
    }
  }, 'isVisible'),
```

This PR gives the user a chance to supply their own slideUp/slideDown, fadeIn/fadeOut etc. functionality when the isVisible property changes.