'set' and relative operations for properties in HSV space

`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.