docs: 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)