Blacklist groups

[disconsis]

Similar to the automatic blacklisting of the fg colors associated with the Normal highlight group, it would be great if there were a variable of blacklisted groups. I felt a need for this when rainbow_parentheses.vim chose the fg color of comments for the parens, making it quite hard to see (with colorscheme base16-onedark). I could add the color to the blacklisted ones, but I think an option that would work across colorschemes would be better.