Add option for enabling local folding settings

Ref #321

doc/vimwiki.txt

@@ -2321,6 +2321,8 @@ Value           Description~
 'expr'          Folding based on expression (folds sections and code blocks)
 'syntax'        Folding based on syntax (folds sections; slower than 'expr')
 'list'          Folding based on expression (folds list subitems; much slower)
+'custom'        Leave the folding settings as they are (e.g. set by another
+                plugin)
 
 Default: ''
 

plugin/vimwiki.vim

@@ -167,6 +167,8 @@ function! s:setup_buffer_enter() "{{{
   elseif g:vimwiki_folding ==? 'syntax'
     setlocal fdm=syntax
     setlocal foldtext=VimwikiFoldText()
+  elseif g:vimwiki_folding ==? 'custom'
+    " do nothing
   else
     setlocal fdm=manual
     normal! zE