Add possibility to pause transition

Hi !
It would be very useful to have the possibility to pause / resume transitions. 
This is a first "naive" go.

`d3.transition.pause()` will pause all active transitions, while `d3.transition.resume` does what it should !
There is also a way to act just on specific transition (transition.pause() and transition.resume())

Use case : a small animation utility written with d3.js - see here (draft) https://www.e-smile.org/solutions/whatIsIt
Cheers,
C.