Remove nested links in HTML output #32

This is a long-standing problem with the XML output, which I won't address, but we have the opportunity to do something reasonable in the HTML output. Basically, <a> tags in HTML are not allowed to have <a> tags in their body. So we should keep some local state around, I suppose, that lets us know whether we are inside a hyperlink and if so, not emit any further <a> tags.