New PIs for inserting prev, next, and TOC URLs

Added new `<?prev_url?>`, `<?next_url?>`, and `<?toc_url?>` PIs for inserting previous, next, and TOC URL text, respectively, into chunked Web output. (These are an alternative to the existing `<?prev_link?>`, `<?next_link?>`, and `<?toc_link?>` PIs, which insert a full `<a>`).