Fix: VimwikiTOC is broken against headers with link (Issue #182)

Problem: VimwikiTOC and follow_link do not support header (anchor) with
link in their body

Solution:
- VimwikiTOC (easy): Create function base.vim s:clean_header_text that
converts [DESC](URL) -> DESC
- follow_link (hard):
-- [[URL]]: was already working due to punctuation removal.
-- [DESC](URL): Search for a potential `]([^)]*)` after every character

doc/vimwiki.txt

@@ -3743,6 +3743,7 @@ Changed:~
 Removed:~
 
 Fixed:~
+    * Issue #182: VimwikiTOC support headers with link
     * Issue #813: iMap <Cr> interfere with completion (pum)
     * Issue #709: Support inline code spans inside emphasis
       Refactoring code, del, eq, sup, sub as regions