Disabling with css classes

Hi! We're using draggabilly at Think Through Math and *love* it, but enabling and disabling it was a little tough because of the need to keep track of a dom element's corresponding draggabilly object.

We do a lot of class toggling to control functionality, so it would be a lot more consistent and a lot easier for us to be able to control whether an element was draggable or not via a class on the element.

I totally understand if adding this feature adds more complexity than you want to, but we're going to be using this change internally and we love contributing back when we can. 

I'm also up for making any modifications or improvements, just let me know!!!

Thank you for this library!!!! :heart_eyes_cat: :heart_eyes_cat: :heart_eyes_cat: