Add Draggabilly support

[MaxwellRebo]

This adds Draggabilly support by default.

If you'd rather this was optional, can do a separate PR for that.

[efournival]

Hi,
I know this is an old PR but this is not working when adding elements dynamically.
You are creating a new Dragabilly instance each time performLayout is called, which cause multiple instances of Dragabilly being binded when the grid is updated. It causes the layout to be completely messed up with strange gaps.
My solution was to only make new elements draggable:

        performLayout: function() {
            var diff = this.diffDomChildren();

            if (diff.removed.length > 0) {
                this.packery.remove(diff.removed);
            }

            if (diff.appended.length > 0) {
                this.packery.appended(diff.appended);
                diff.appended.forEach( this.makeEachDraggable );
            }

            if (diff.prepended.length > 0) {
                this.packery.prepended(diff.prepended);
                diff.prepended.forEach( this.makeEachDraggable );
            }

            this.packery.reloadItems();
            this.packery.layout();
        },

[MaxwellRebo]

Let me have a look shortly. Do you mind if I use your solution and compare?

[efournival]

Yeah of course, let's say the code I gave above is public domain.