Property Color unset invalid value

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`

Start at the cssutils.profile.validate entry point and reproduce the reported check with the color property and the value 'unset'. Read the surrounding color-value validation rules to determine how this case is handled. Done means the validation returns True for this input without changing the expected behavior for other color values.

Written by the indexing model from the issue text.