Accessing velocity in children-function

Hi Cheng
Firstly, Awesome library! I've been having a lot of fun with it the past few days :smile:

So, I had the idea that the current velocity of a animated property is a nice value to animate specific css-properties with. However I haven't been able to acces the velocity using the current api. (Maybe, I have just overlooked something.)

So I forked react-motion and crudely exposed the velocities in StaggeredMotion's children-function as a second argument and tried driving translateZ and box-shadow with the current velocity
I'm pretty happy with the [result](https://b0513122f4e33807f0e8db82fdcbf846d3cad11e-www.googledrive.com/host/0BwUo2j-YQKKoRUNiSzhBMkFfMVE), but I feel like my approach is too difficult..

I'd love it to have your opinion on this. :)

Start by reading the current StaggeredMotion children-function API and compare it with the issue author's forked behavior. Done means the current velocity is exposed through a documented API and can be used to drive related animated properties.

Written by the indexing model from the issue text.