Initializes markdown extension components.

def extendMarkdown(self, md, md_globals=None):
        """Initializes markdown extension components."""
        if any(
                x not in md.treeprocessors
                for x in self.REQUIRED_EXTENSION_INTERNAL_NAMES):
            raise RuntimeError(
                "The attr_cols markdown extension depends the following"
                " extensions which must preceded it in the extension"
                " list: %s" % ", ".join(self.REQUIRED_EXTENSIONS))
        processor = AttrColTreeProcessor(md, self.conf)
        md.treeprocessors.register(
            processor, 'attr_cols',
            5)