Method has_default

Source/Utils/cxxopts.hpp:1375–1380

TODO: maybe default options should count towards the number of arguments

    // TODO: maybe default options should count towards the number of arguments
    CXXOPTS_NODISCARD
    bool
    has_default() const noexcept
    {
      return m_default;
    }

    template <typename T>
    const T&