Expose thumb index in change handlers

[axelboc]

I'm using a slider with two thumbs and using it as a controlled component with value and onChange. It would be quite helpful if I had an easy way of knowing which thumb has moved in my onChange and onAfterChange event handlers (and onBeforeChange as well, why not):

onChange={(value, index) => { ... }}

In most cases, a workaround is to do an equality check for each value, of course, but if the initial values are not multiples of step, they get rounded, which makes things a bit more challenging.