Support `deg` notation for hsl & hwb

<hue> can be an integer (considered as a degree) or an angle (which can
be other things than a degree but it’s better than nothing to read
`deg` right ?)

References:
- http://dev.w3.org/csswg/css-color/#the-hsl-notation
- http://dev.w3.org/csswg/css-color/#the-hwb-notation
- http://dev.w3.org/csswg/css-values-3/#angle-value