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

plugin/tagbar.vim

@@ -101,6 +101,7 @@ function! s:setup_options() abort
         \ ['position', default_pos],
         \ ['previewwin_pos', previewwin_pos],
         \ ['scopestrs', {}],
+        \ ['scrolloff', 0],
         \ ['show_balloon', 1],
         \ ['show_visibility', 1],
         \ ['show_linenumbers', 0],