Custom animations support

[subbotkin]

Currently It's unavailable to make an extension to AnimatedSink with custom animation like this:

extension AnimatedSink where Base: MyView {
   public func customAnimation() -> AnimatedSink<Base> {
        let type = AnimationType<Base>(duration: duration, setup: { view in
            //...
        }, animations: { view in
            //...
        })
        return AnimatedSink<Base>(base: self.base, type: type)
    }
}

because AnimationType and AnimatedSink initializers and AnimatedSink base variable has internal protection level.

[icanzilb]

Merging and pushing a release in few minutes, thanks again