Add support for generating heading anchors in HTML output.

I need to support generating anchors in the HTML output, similar to the way GitHub allows the linking to each heading.  I don't know if this is in your roadmap, but I thought I would provide a pull request in case you wanted to utilise it.

With the `anchors: true` for the marked options, it will take the heading text and turn it into a valid `name` attribute value.  It does assume that all heading text is unique.