Add hwb() support

[MoOx]

I've added hwb support for color lib (to bring whiteness & blackness for css-color-functions)
So this brand new plugin (made by @ddprrt) is available & using this new implementation https://github.com/reworkcss/rework-hwb

Looks good to you @ianstormtaylor ?

• support in rework-color-function
• adding rework-hwb

[ianstormtaylor]

Yeah that looks sweet! Nice idea

[MoOx]

rework-hwb will be easy to add, but for rework-color-function we still need to get improvements from ianstormtaylor/css-color-function#4