Add onHandleClick prop

[mjcarlin]

I'd like to be able to change the color of a handle when it has been clicked. Here is my use case:

I have configured ReactRanger with one handle. To the left of the handle the track is colored red to indicate "bad" values and to the right of the handle the track is colored green to indicate "good" values. I want to toggle the handle color between red and green each time the handle is clicked. This allows the user to indicate if the handle value itself should represent "bad" or "good".

Thanks.