Support for PIs for navigating among HTML chunks

The HTMLBook stylesheets now support the following PIs:

````xml
<?prev_link?>
<?next_link?>
<?toc_link?>
````

Which you can use in either body text or in a custom.chunk.wrapper file for chunked HTML to insert links that go to the previous HTML file in the sequence, the next HTML file in the sequence, and the TOC file, respectively.