Tweening between arbitrary shapes instead of an existing shape

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Is your feature request related to a problem? Please describe.
I'm always frustrated when I can't change the tweened shape much (e.g. by adding/removing control points, lines, etc.).

Describe the solution you'd like
I'd like tweening between arbitrary objects. I think https://github.com/veltman/flubber could be used for this.

Describe alternatives you've considered
Draw all shapes I want to tween between. Choose the most complex one. And try to recreate all others using only the lines, points, etc. from the most complex one. This is enormously difficult, time consuming, and the result is often not quite as good. It's also one-way thing not allowing any further changes (e.g. adding yet more complex shape somewhere into the tween chain).

Additional context
This could potentially solve #54 as a side effect.