Add an instance of FigcaptionProcessor to BlockParser.

def extendMarkdown(self, md, md_globals):
        """ Add an instance of FigcaptionProcessor to BlockParser. """

        # def_list = 'def_list' in md.registeredExtensions
        md.parser.blockprocessors.add(
            'figcaption', FigcaptionProcessor(md.parser), '<ulist')