Use constants for colours

[dmitmel]

I've also defined a macro called define_colour, it can be used like this:

// The colour struct should be available in the current scope for this macro to work!
use graphics::colour::Colour;
define_colour!(SOMETHING, 1, 2, 3);

And it will generate the following code:

const SOMETHING: Colour = Colour { r: 1, g: 2, b: 3 };

[dmitmel]

@Hopson97 I'll rebase this branch onto master

[dmitmel]

@Hopson97 Done, this branch now can be safely merged