Ideas for px:html-chunker

Some new options could be added to `px:html-chunker`:

- [ ] the "always-break-before" and "always-break-after" options passed to `px:chunker` are currently hard-coded but could be exposed
- [ ] option to insert "prev" and "next" links
- [ ] option(s) to update the heading levels of the chunks using the new [`px:html-outline`](https://github.com/daisy/pipeline-modules/issues/14)

In addition, in the lower-level `px:chunker` step, the "link-attribute-name" option should be replaced by a "mapping" output that contains a `d:fileset` document with a mapping from old to new file names (similar to `px:fileset-copy` and `px:epub3-safe-uris`). A separate `px:html-update-links` step can then be used to fix the attributes. (Done in aaa3a996cbbd2d366d2daf22dfe46761532ad200.)