make the delimitermap a global variable

This allows people to edit the delimiters at run time.

The option NERDCustomDelimiters can be used in a vimrc (or similar) but
is not useful after vim has loaded. So just open up the s:delimiterMap
hash.

plugin/NERD_commenter.vim

@@ -406,6 +406,8 @@ let s:delimiterMap = {
     \ 'z8a': { 'left': ';' }
     \ }
 
+let g:NERDDelimiterMap = s:delimiterMap
+
 if exists("g:NERDCustomDelimiters")
     call extend(s:delimiterMap, g:NERDCustomDelimiters)
 endif