Add highlight arguments (e.g. bold) support to flatten.

autoload/lightline/colorscheme.vim

@@ -211,7 +211,14 @@ function! lightline#colorscheme#flatten(p) abort
   for k in values(a:p)
     for l in values(k)
       for m in range(len(l))
+        let attr = ""
+        if len(l[m]) == 3
+          let attr = l[m][2]
+        endif
         let l[m] = [l[m][0][0], l[m][1][0], l[m][0][1], l[m][1][1]]
+        if !empty(attr)
+          call add(l[m], attr)
+        endif
       endfor
     endfor
   endfor