Add support for level 6 sections and generalize Sink API for sections

XWiki supports 6 levels (see http://platform.xwiki.org/xwiki/bin/view/Main/XWikiSyntax) and thus I cannot write a valid parser if the Sink API doesn't support a level 6.

I'd even suggest to unify all sink method for sections into a single method with a level parameter passed instead of having sectionTitle1(), etc.