Help visibility (#686)

Closes #410

Add `g:tagbar_help_visibility` option to always show the help dialog

plugin/tagbar.vim

@@ -97,6 +97,7 @@ function! s:setup_options() abort
         \ ['height', 10],
         \ ['indent', 2],
         \ ['left', 0],
+        \ ['help_visibility', 0],
         \ ['position', default_pos],
         \ ['previewwin_pos', previewwin_pos],
         \ ['scopestrs', {}],