:bug: Fix number followed by = matched as stylus variable
:lipstick: Use a more meaningful color in expressions comments

Using black as demonstration is not a good thing as non parsable colors
appears as black as well.
Add support for the HWB color model

Includes:
- Conversion from and to rgb and hsv models
- A `hwb` accessor on colors

Closes abe33/atom-color-highlight#20
Add support for the opacity argument of the hwb function
Add support for the css4 `gray` functional notation
:memo: Update CHANGELOG
Prepare 1.9.0 release