Add 'headerid' markdown extension.

Just a simple addition to the extension list for rendering markdown.

Currently, it's impossible to link to a URL of the form `http://some-site.com/whatever#relevant_header`, unless you use raw HTML in your markdown to specify the id attribute. This is very messy in the context of a markdown document.

The extension is not intrusive and shouldn't mess up anything for anyone. The default algorithm for generating ids is to slugify the header text, so `Cool Header Text` becomes `cool-header-text`.