ENH: Add 'describe extension' to SlicerWizard.py

Add an action to "describe" an extension (i.e. dump its generated
description content to stdout). This is useful both as a user check
before publishing, and potentially can be used by CMake as an alternate
means of generating the s4ext file for the build.

Note that at the moment the extension must have a git repository.
Eventually we want to support subversion repositories and also
extensions that don't yet have a repository, but a number of places will
need to change for that (e.g. publishing should also support svn).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22972 3bd1e089-480b-0410-8dfb-8563597acbee