Add an optional up arg to Transform.look_at

[stasm]

#45 added Entity.look_at which currently always assumes that the look-at rotation should align the object's left direction with the world`s horizontal plane.

This might be helpful: https://stackoverflow.com/questions/8942950/how-do-i-find-the-orthogonal-projection-of-a-point-onto-a-plane

[michalbe]

This logic is now in Transfrom component.