Refactored the command line config system and other aspects of config. * Removed ``NoConfigDefault`` from our config logic entirely. It turns out that argparse has a nice ``argparse.SUPPRESS`` object that handles this nicely (r1283, r1284). * Completely cleared the previous config when :meth:`load_config` is called to allow it to be called more than once. * Added some additional tests and docs for the config system. * Removed the ``override_config`` and ``constructor_config`` config stages from :class:`~IPython.core.application.Applications` (r1293). * Completely refactored the command line config parser classes. We are now using a more object oriented design that allows for fine grained inheritance and good encapsulation.