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

autoload/tagbar.vim

@@ -1967,7 +1967,12 @@ function! s:PrintKinds(typeinfo, fileinfo) abort
             endif
 
             let padding = g:tagbar_show_visibility ? ' ' : ''
-            call add(output, foldmarker . padding . kind.long)
+            if g:tagbar_show_tag_count
+                let tag_count = ' (' . len(curtags) . ')'
+                call add(output, foldmarker . padding . kind.long . tag_count)
+            else
+                call add(output, foldmarker . padding . kind.long)
+            endif
 
             let curline                   = len(output) + offset
             let kindtag.tline             = curline

doc/tagbar.txt

@@ -718,6 +718,18 @@ window. It can be set to the following values:
 Example:
 >
     let g:tagbar_show_tag_linenumbers = 1
+<
+                                                     *g:tagbar_show_tag_count*
+g:tagbar_show_tag_count~
+Default: 0
+
+This option allows showing the tag count next to the tag kind in the tagbar
+window. This will show up like this >
+   > functions (<tag-count>)
+<
+Example:
+>
+    let g:tagbar_show_tag_count = 1
 <
                                                      *g:tagbar_hide_nonpublic*
 g:tagbar_hide_nonpublic~

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],