Method operator[]

tests/catch.hpp:7279–7282

        Opt( T &ref, std::string const &hint ) : ParserRefImpl( ref, hint ) {}

        auto operator[]( std::string const &optName ) -> Opt & {
            m_optNames.push_back( optName );
            return *this;
        }

        auto getHelpColumns() const -> std::vector<HelpColumns> {
            std::ostringstream oss;