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

syntax/vimwiki.vim

@@ -141,7 +141,7 @@ else
 endif
 
 
-" Weblink
+" Weblink [DESCRIPTION](FILE)
 call s:add_target_syntax_ON(vimwiki#vars#get_syntaxlocal('rxWeblink'), 'VimwikiLink')