Always invoke the root command as `whipper`

[CommandMC]

When installing from source & launching using python -m whipper, sys.argv[0] ends up as <path to source checkout>/whipper/__main__.py. In turn, __main__.py.foo.bar is used for lookups in the config file, instead of whipper.foo.bar, which means the config file is only partially respected when running from source

Resolve this by hardcoding whipper as the program name