Having only !--no-flag specified when adding an option or having it as the first name will result in wrong (reversed) config file outputs.
Steps to reproduce:

1. Run with --no-flag. This results with setting the variable flag=false (correct), but it outputs no-flag=false.
2. Save the output (no-flag=false) to config.toml
3. Run with --config "path/to/config.toml". This results with flag=true and the output is no-flag=true.

#include <CLI11/CLI11.hpp>

#include <iostream>

int main()
{
  CLI::App app;
  bool flag;
  app.add_flag("!--no-flag", flag);
  app.set_config("--config,--ini");
  try {
    app.parse(argc, argv);
  } catch (const CLI::Error& e) {
    return app.exit(e);
  }

  std::cout << app.config_to_str(false, false) << std::endl;
  return 0;
}