code: def add_simple_formatter(self, tag_name, format_string, **kwargs):
          
          def _render(name, value, options, parent, context):
              fmt = {}
              if options:
                  fmt.update(options)
              fmt.update({'value': value})
              return format_string % fmt
          self.add_formatter(tag_name, _render, **kwargs)
docs: Installs a formatter that takes the tag options dictionary, puts a value key
          in it, and uses it as a format dictionary to the given format string.