If optional dependencies are not included, feature is simply ignored.

```javascript
if ( this.options.draggable && $.fn.draggable ) {
    this._makeDraggable();
}
if ( this.options.resizable && $.fn.resizable ) {
    this._makeResizable();
}
```

If we are not requiring these two dependencies, shall we throw an error if user picks a feature whose dependency is not loaded (eg. `this.options.draggable && !$.fn.draggable`)?