deploy-ng: Allow platforms to be specified on the command line

This makes it easier to build for one platform even if multiple
platforms are specified in setup.py/cfg. For example:

python setup.py build_apps -p win_amd64

Or, for multiple platforms:

python setup.py build_apps -p win32,win_amd64

direct/src/showutil/dist.py

@@ -189,6 +189,7 @@ class build_apps(setuptools.Command):
     user_options = [
         ('build-base=', None, 'directory to build applications in'),
         ('requirements-path=', None, 'path to requirements.txt file for pip'),
+        ('platforms=', 'p', 'a list of platforms to build for'),
     ]
     default_file_handlers = {
         '.egg': egg2bam,