Add generation of @xtrf and @xtrc in the XSLT step

[ColinMaudry]

Since I making preprocessing optional #57, I will lose the benefit of the pregeneration of the @xtrf and @xtrc attributes. I consequently need to port their generation from wherever it's done to dita2rdf.xsl.

[ColinMaudry]

OK, making preprocess optional is complicated. Postponed.