Function registers into self.commands from module extension.
        All extension subcommands are registered using the name convention
        'extension:command'

        Example:
            If you have a redis extension namely 'gredis', the extension
            commands can be accessed by the following;

            $ python glim.py gredis:ping

        Args
        ----
          module (module): The module name.
          extension (string): The extension name.

def register_extension(self, module, extension):
        """
        Function registers into self.commands from module extension.
        All extension subcommands are registered using the name convention
        'extension:command'

        Example:
            If you have a redis extension namely 'gredis', the extension
            commands can be accessed by the following;

            $ python glim.py gredis:ping

        Args
        ----
          module (module): The module name.
          extension (string): The extension name.
        """
        if module is not None:
            cmds = self.retrieve_commands(module)
            commands = []

            for c in cmds:
                if self.valid_name(c.name):
                    name = '%s:%s' % (extension, c.name)
                    cmd = c(self.subparsers, name)
                    self.commands.append(cmd)