Support a function for end values.

I propose support for a function as the first parameter for the Tween.to()-method. The function is passed a copy of the ```_object``` as parameter and after any calculations needs to return an object with the properties for _endValues.

This enables an easy way to create reusable tween chains that can dynamically take into account the start position of any of the tweens in the chain.

One example of such could be an object that moves to where the user clicks, then moves some 50 pixels to the right, out of the way while still avoiding going outside the containing object.

I think this could be done with existing callbacks, but for my use, this seemed just too elegant to leave unimplemented.