Property Color unset invalid value

[Thokas]

I'm only using this Package to validate CSS-Values, but I've stumbled upon the case where a color value is set to 'unset' and the profile validation (cssutils.profile.validate) returns a false for this.

Expected Behavior

cssutils.profile.validate('color', 'unset') is True

Actual Behavior

cssutils.profile.validate('color', 'unset') is False

[jaraco]

Looks like unset is a legitimate setting. I welcome a patch to fix it.