Add g:tagbar_scrolloff configuration (#692)

* Add g:tagbar_scroll_offset configuration
Closes #564

Add option for `g:tagbar_scrolloff` to issue a `setlocal scrolloff=#` during tagbar window init

* Address review comments

autoload/tagbar.vim

@@ -947,6 +947,10 @@ function! s:InitWindow(autoclose) abort
     setlocal nomodifiable
     setlocal textwidth=0
 
+    if g:tagbar_scrolloff > 0
+        execute 'setlocal scrolloff=' . g:tagbar_scrolloff
+    endif
+
     if g:tagbar_show_balloon == 1 && has('balloon_eval')
         setlocal balloonexpr=TagbarBalloonExpr()
         set ballooneval