BUG: Fix argparse assert wrapping arguments

Tweak WizardHelpFormatter in SlicerWizard.py to temporarily replace
'[]'s in option value strings during formatting in order to avoid
argparse splitting these and tripping an assert in its internals.
ENH: Add user templates to SlicerWizard.py

Add support for user-specified template directories and keys to
SlicerWizard.py. This allows users to provide their own templates in
addition to the "built in" templates.