'set' and relative operations for properties in HSV space

[sschuldenzucker]

pastel set doesn't support properties of HSV space and there is no operation that supports relative changes in HSV space (darken and desaturate operate in HSL)

I've been looking for this b/c I've found it quite convenient to reduce V while keeping H and S the same to create darker / hierarchically subordinate versions of a given color for UI text.

pastel already supports this "manually" using pastel format hsv, manually changing the result and reading it back in to pastel color, so it should be pretty straightforward.