Registers a flag whose value can be any string.

def DEFINE_string(  # pylint: disable=invalid-name,redefined-builtin
    name, default, help, flag_values=_flagvalues.FLAGS, **args):
  """Registers a flag whose value can be any string."""
  parser = _argument_parser.ArgumentParser()
  serializer = _argument_parser.ArgumentSerializer()
  DEFINE(parser, name, default, help, flag_values, serializer, **args)