Add 2D sugar for scale

[paulyoung]

I believe scale-x and scale-y will be introduced in v2.1 of the engine.

This pull request allows:

scale: = 1 - number / 10;

as convenience for:

scale-x: = 1 - number / 10;
scale-y: = 1 - number / 10;

[Inviz]

I thought of implementing it as expansion in engine, but i was holding it off as not very important. Another quirk with scale is that it's actually a 3d property and also implies scale-z...

But i like how much more trivial it is in parser.

[paulyoung]

Maybe the 2D unpacker should be made more generic and combine any number of properties.

[paulyoung]

Alternatively:

scale-xy: ["scale-x", "scale-y"]
scale: ["scale-xy", "scale-z"]