#449 Add index.preferred.page.properties to epub/xhtml

[David Gardiner]

Please add an attribute set "index.preferred.page.properties" to epub, HTML, XHTML, XHTML1.1 and HTML5 stylesheets. This attribute set exists only in FO. There is currently IDPF work to develop indexing standards for epub 3.0 so this should be a feature incorporated in anticipation of new standards.