Support XY-scaling on linear_extrude (resolves #268)

[FlyingGaz]

This pull request resolves #268.

The scale parameter of linear_extrude now supports (ℝ, ℝ) and ℝ -> (ℝ, ℝ) in addition to ℝ and ℝ -> ℝ. This allows x and y to be scaled independently from each other while extruding.

Example demonstrating all 4 variants:

union() {
    linear_extrude(height = 10, scale = 2)
    square(10, center = true);

    translate([30, 0, 0])
    linear_extrude(height = 10, scale(z) = 1 + z / 10)
    square(10, center = true);
    
    translate([0, 30, 0])
    linear_extrude(height = 10, scale = [2, 1.5])
    square(10, center = true);
    
    translate([30, 30, 0])
    linear_extrude(height = 10, scale(z) = [1 + z / 10, 1 + z / 20])
    square(10, center = true);
}

Feedback and suggestions welcome.

[julialongtin]

Looks good. I'm not sure i like the naming, i might go over that in the future, but.. thanks!