Add optional vim-css-color package

pkgs/vim/default.nix

@@ -118,6 +118,7 @@ let
       ] ++ extraPlugins;
       # manually loadable by calling `:packadd $plugin-name`
       opt = [
+        vim-css-color
       ];
       # To automatically load a plugin when opening a filetype, add vimrc lines like:
       # autocmd FileType php :packadd phpCompletion