Swizzle support for colors

[DigiDuncan]

We talked about this in the Discord once. And why not, right? It's a cheap function to write.

Could probably have docs or something!

Takes a string of r, g, b, and a's and returns a tuple of those properties in order.

It's an OpenGL thing.

[pushfoo]

This is almost ready. I think the current implementation is faster and easier than __getattribute__ hacks, even if color_instance.brga would be cleaner code.

Changes I'd like:

1. Use ValueError insteaad of Exception
2. Consider mentioning the offending value in the exception
3. Add tests

[pushfoo]

This looks good now.