Support nested color function

Fix parsing issue causing "Missing closing parentheses" even if it
looks fine.
Also, better error reporting when possible.

Should close https://github.com/ianstormtaylor/rework-color-function/issues/4
Add whiteness & blackness (methods using hwb() channel)

Close #1
Fix #2
Merge pull request #4 from MoOx/master

Support nested color function + Add whiteness & blackness