Add g:tagbar_show_balloon option

autoload/tagbar.vim

@@ -758,7 +758,7 @@ function! s:InitWindow(autoclose) abort
     setlocal nomodifiable
     setlocal textwidth=0
 
-    if has('balloon_eval')
+    if g:tagbar_show_balloon == 1 && has('balloon_eval')
         setlocal balloonexpr=TagbarBalloonExpr()
         set ballooneval
     endif