BUG: Associate "--cmd" with installed launcher on windows

To start a console with the Slicer environement, from an installed tree,
instead of doing:

  .\Slicer.exe --launch "C:/Windows/System32/cmd.exe" /c start cmd

user will be able to simply do:

  .\Slicer.exe --cmd

Fixes #2917

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22295 3bd1e089-480b-0410-8dfb-8563597acbee