Add support for xref/@xrefstyle (Issue 414)

[fsteimke]

This is my very first pull request, i hope i am doing it right.

Changes in link.xsl and xref.xsl for a minimal support of xref/@xrefstyle (Issue 414).

If an xrefstyle attribute exists, a l:template Element is created on the fly by analyzing the xrefstyle content, and applied with a new tp:apply-xref-template Template. Only two of the Template %-letter substitutions from Table 4.1 in the reference Manual are supported:

• %c (content) gives an lt:contentElement;
• %l (label) gives an lt:label Element
• Anything else is wrapped in an lt:text Element.

Greetings, Frank

[fsteimke]

OK, i see that i have to learn more about contributing code. I will close this pull request and try to do better next time.