Support for nested draggable/sortable pattern with isAccepted callback

Ryan, I've been using your knockout-sortable plugin in a large project that I'm working on and I found the need to add some supporting code for a nested draggable/sortable pattern where each view model can determine whether it accepts the dragged item or not. I've implemented this and also created an associated jsfiddle with the pattern of the intended use. The overall changes are around making the clone callback take the view model that you are adding to as a parameter and adding an isAccepted callback that works similarly (delegates the responsibility of whether the target accepts the element to the view model of the target). Take a look if you get a chance: http://jsfiddle.net/dflor003/hBv4p/