def argument(value, options = {})
      return unless @shell.applicable?(options)
      value = value.to_s
      if @shell.requires_escaping?(value)
        value = @shell.escape_value(value)
      end
      @command << ' ' << value
      @last_arg = :argument
    end
def argument(value, options = {})
      return unless @shell.applicable?(options)
      value = value.to_s
      if @shell.requires_escaping?(value)
        value = @shell.escape_value(value)
      end
      @command << ' ' << value
      @last_arg = :argument
    end
Add an unquoted argument to the command.
 This is not useful for commands executed directly, since the arguments
 are note interpreted by a shell in that case.