Add "sample" subcommand and predefined color scales?

[sharkdp]

This could be incredible useful for shell scripts that want easy access to perceptually uniform color scales.

pastel sample viridis 0.34
pastel sample blueToRed 0.2

Note: this would be quite similar to pastel mix blue red -f ... and also pastel gradient blue red. We should try to come up with a consistent API.

Basically, pastel gradient red blue ~ pastel sample redToBlue 0.0; pastel sample redToBlue 0.1; ....

See: http://sharkdp.github.io/purescript-colors/#scale