find any installed plugins and register them.

def register_plugins():
    """find any installed plugins and register them."""
    if pkg_resources:  # pragma: no cover
        for ep in pkg_resources.iter_entry_points('slam_plugins'):
            plugin = ep.load()

            # add any init options to the main init command
            if hasattr(plugin, 'init') and hasattr(plugin.init, '_arguments'):
                for arg in plugin.init._arguments:
                    init.parser.add_argument(*arg[0], **arg[1])
                init._arguments += plugin.init._arguments
                init._argnames += plugin.init._argnames

            plugins[ep.name] = plugin