Add g:tagbar_show_tag_count option (#685)

* Add g:tagbar_show_tag_count option

Closes #290

This option will show the tag kind count next to the kind label in the
tagbar window.

* Address review comments

plugin/tagbar.vim

@@ -103,6 +103,7 @@ function! s:setup_options() abort
         \ ['show_balloon', 1],
         \ ['show_visibility', 1],
         \ ['show_linenumbers', 0],
+        \ ['show_tag_count', 0],
         \ ['show_tag_linenumbers', 0],
         \ ['singleclick', 0],
         \ ['sort', 1],