Adding color handling

[FayCarsons]

My first attempt at a system for coloring shapes and setting background in init. Shapes now have a color field that is an 8bit RGB three-tuple. Shape constructors are the same, inserting black as default, but shapes can be piped to a new function with_color that returns a new shape with the color field set to the arg.

So creating a gray circle can look like this:

let c = circle 50 |> with_color (128, 128, 128)

[nikochiko]

Thanks! This looks great!

I have suggested some renaming and refactor changes. Please go through the review.

[FayCarsons]

Applied changes, caught a couple of small issues, and added color mapping functions: map_color which applies a fn to both stroke and fill, and map_stroke/map_fill which apply a fn to specific color fields in a shape.

[FayCarsons]

Fixed merge conflicts!