Allow customization of header level of TOC

Instead of forcing the TOC to always be at header level 1, allow the
user to specify via the option g:vimwiki_toc_header_level what level
they want.

This defaults to 1, so if the user does nothing then the old behavior
will remain the same.

doc/vimwiki.txt

@@ -2872,6 +2872,15 @@ appropriate word in your mother tongue like this: >
 The default is 'Contents'.
 
 
+------------------------------------------------------------------------------
+*g:vimwiki_toc_header_level*
+
+The header level of the Table of Contents (see |vimwiki-toc|). Valid values
+are from 1 to 6.
+
+The default is 1.
+
+
 ------------------------------------------------------------------------------
 *g:vimwiki_map_prefix*