Parameterized handling for "pagenum" classes for print outputs

This pull request adds handling to generate "pagenum" class values on designated XREFs for paged outputs (print/Web PDF) in order to facilitate CSS Paged Media styling to add page number gentext (via the `target-counter()` function: http://dev.w3.org/csswg/css-gcpm/#target-counter).

The HTMLBook XSL now has available a parameter called `xref.elements.pagenum.in.class`, which takes a newline-separated list of elements (element names are "semantic" names defined in the `semantic-name` template in common.xsl), e.g.:

````xml
  <xsl:param name="xref.elements.pagenum.in.class">
sect1
sect2
sect3
sect4
sect5
sidebar
</xsl:param>
````

 Any XREFs pointing to an element in this list will have `pagenum` added to their `class` attribute value.