Extension for GFM strikethrough using ~~ (GitHub Flavored Markdown). <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 strikethrough text regions are turned into {@link Strikethrough} nodes. </p>