Extension for YAML-like metadata. <p> Create it with {@link #create()} and then configure it on the builders ({@link hunt.markdown.parser.Parser.Builder#extensions(Iterable)}, {@link HtmlRenderer.Builder#extensions(Iterable)}). </p> <p> The parsed metadata is turned into {@link YamlFrontMatterNode}. You can access the metadata using {@link YamlFrontMatterVisitor}. </p>