Footnoteref handling

Added "footnoteref" handling to htmlbook-xsl. Footnoterefs are used when you want to have two footnote markers in the body text pointing to the same footnote (one of the markers is formatted as a "footnoteref" XREF that points to the actual footnote).

Markup used is the following:

````html
<a data-type="footnoteref" href="#id_of_footnote_being_referenced"/>
````