def option(name)
      config = chain_options[name] || raise_no_option_error(name)
      Option.new(config).tap { |o| o.initial_value(values[name]) if values.key?(name) }
    end
def option(name)
      config = chain_options[name] || raise_no_option_error(name)
      Option.new(config).tap { |o| o.initial_value(values[name]) if values.key?(name) }
    end
Returns an option registered under `name`.