Add g:tagbar_highlight_follow_insert option (#725)

This adds an option making the highlight in the Tagbar follow the cursor
in insert mode as well. This feature is guarded behind an option because
it may introduce some delay in the input.

GitHub: closes #724

plugin/tagbar.vim

@@ -100,6 +100,7 @@ function! s:setup_options() abort
         \ ['jump_lazy_scroll', 0],
         \ ['left', 0],
         \ ['help_visibility', 0],
+        \ ['highlight_follow_insert', 0],
         \ ['highlight_method', 'nearest-stl'],
         \ ['ignore_anonymous', 0],
         \ ['position', default_pos],