Don't warn for units on hsl(0, 0%, 0%)

Saturation and luminosity are incorrectly reported as having optional zero units:
```css
color: hsl(0,0%,0%);
```
This gives "Best Practice: Don't specify the unit type (%) when the value is zero" twice; removing the % creates invalid CSS (and isn't even recognized by your neat color features)

This may not be the most elegant way to add this check.