support set contrast with ratio

[gasolin]

as w3c spec of CSS filter effects
https://www.w3.org/TR/filter-effects-1/#contrastEquivalent

CSS user able to set filter: contrast(100) property to change the color, ex:
https://vanseodesign.com/css/filters-to-adjust-brightness-contrast-opacity-and-inversion/

Another nice to have is set the brightness via https://github.com/Qix-/color/issues/53#issuecomment-487822576

[Qix-]

Hi - this doesn't make much sense as the contrast filters require an entire image in order to determine the relative adjustments per-pixel. This library just works with single colors.