STYLE: Improve SlicerWizard doc generation

Pass Slicer version and wiki version into SlicerWizard documentation
generation, and make use of the same. Add 'author' option and use as
required by the various documentation types, rather than each
duplicating the string (which was still set to "Author").

The way we arrange to communicate values into conf.py is a little
esoteric, but it works (and thus saves us from needing to make conf.py a
configured file), and seems to be the only reasonable way to pass
information from the command line to conf.py.