New <?prev_url?>, <?next_url?>, and <?toc_url?> PIs for inserting just a URL, rather than a full <a>.
Escaped version of <?prev_url?>, <?next_url?>, and <?toc_url?> for use in href attrs.
Merge pull request #161 from oreillymedia/urls

New PIs for inserting prev, next, and TOC URLs