docs: def set_auto_handler(self, flag, handler):
          """Set pre-auto-handler for a flag.
  
          the handler must accept two argument: first the `pie` which
          referent to the current `Docpie` instance, second, the `flag`
          which is the flag found in `argv`.
  
          Different from `extra` argument, this will set the alias
          option you defined in `Option` section with the same
          behavior.
          """
          assert flag.startswith('-') and flag not in ('-', '--')
          alias = self.find_flag_alias(flag) or []
          self.extra[flag] = handler
          for each in alias:
              self.extra[each] = handler