Method subcommand

tests/issue_79_run_test.cpp:21–29

struct subcommand
{
	subcommand(lyra::cli & cli)
	{
		cli.add_argument(
			lyra::command("subcommand", [this](lyra::group const &) { exec(); })
				.add_argument(lyra::opt(
					[](std::string const & s) { return always_error_cb(s); },
					"arg")
						.name("--arg")));
	}

	private:
	void exec() { std::cout << "Subcommand\n"; }