Extend slicer version handling to allow differentiation of Release vs Nightly

Slicer version should now be specified as a string of the form:

  Slicer X.Y[-Nightly]

Introduced the following functions:
* getSlicerMajorMinorVersion
* isSlicerReleaseVersion

Function assuming wiki page namespace was 'Nighty' have also updated to
extract the release identifier given the Slicer version. It means, the
pages in either one of these namespaces (Nightly, 4.4, ... ) can now
be updated based on the provided or auto-discoverted Slicer version.

Note that this is a backward incompatible change.
Update getMetadataFiles() function to log more information
Improve 'updateWiki' to either update the regular or the test landing page

If specified, the option '--test-wiki-update' will mean that the
landing page 'User:UpdateBot/Issue-2843-Consolidated-Extension-List'
will be used instead of 'Documentation'.