add 'cubehelix' colour scheme

[safareli]

I came across 'cubehelix' colour scheme.
This page shows how different parameters effect the scheme. it would be great if we can reconstract that configuration in some object and then define something like cubehelix:: CubehelixConfig -> ColorScale.
Here is python implementation, on original page there are cople more.

[sharkdp]

👍