Extension for adding auto generated IDs to headings. <p> Create it with {@link #create()} or {@link #builder()} and then configure it on the renderer builder ({@link HtmlRenderer.Builder#extensions(Iterable)}). <p> The heading text will be used to create the id. Multiple headings with the same text will result in appending a hyphen and number. For example: <pre><code> # Heading # Heading </code></pre> will result in <pre><code> <h1 id="heading">Heading</h1> <h1 id="heading-1">Heading</h1> </code></pre>