Pass follow-on code as a parameter to animate.

Prior to this change, code be run following a completed animation
by setting `.next` on the object returned by the `animate()` call.
This code is now passed as an additional parameter to the
`animate()` call, e.g. instead of:

    THREE.Animatic.animate(e.rotation, {z: 0.0}, 0.25).next = tick1;

you now write:

    THREE.Animatic.animate(e.rotation, {z: 0.0}, 0.25, tick1);
Fixed last commit message.