Proposal: Add cancelation token

[vkondrashkov]

At this moment when animation starts we can't control it in any way. For some reason animation should be canceled and new one should start, but if do it without canceling previous one there would be animation collision.

It would be nice to access cancelation token, for example:

let currentAnimationToken = layer.lotus.runAnimation { make in
    make.scaling.to(2.0).during(1.0)
}.then { make in
    make.rotation.to(6.0 * .pi).during(2.0).delay(0.5)
}

// ...
currentAnimationToken.stopAnimation() // terminate or cancel