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

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~