ui: Add 'rgb' parameter to ui_attach

When set to false, nvim will send cterm color numbers with `highlight_set`.

src/nvim/ui.h

@@ -13,6 +13,7 @@ typedef struct {
 typedef struct ui_t UI;
 
 struct ui_t {
+  bool rgb;
   int width, height;
   void *data;
   void (*resize)(UI *ui, int rows, int columns);