with_toc_data: GitHub style anchors

GitHub uses different anchors for TOC data than redcarpet, which is odd because my understanding is that GitHub uses Redcarpet to render markdown files.

The markdown `# Getting Started` becomes the anchor `<a name="getting-started">` on GitHub, whereas for Redcarpet it becomes `<h1 id="toc_1">`.

Could Redcarpet output both style of id, or have the option of outputting the GitHub style id?

Thanks,
Shaun