add ability to specify 'easing' interpolation rule for animation

[skyebend]

currently it has kind of a stop-and-go feeling, some users may want this, but it doesn't match the movies very closely. add a 'animationRule' option to configure this

[michalgm]

this should be pretty easy, as long as we stick to the built-in d3 easing functions. Or, I suppose we could allow someone to specify their own custom easing function, but they'd probably need to register it with d3 themselves or something. Let me know if we should change the default from d3's cubic-in-out.