Closes #277

[p-ranav]

This PR adds in-built support for .choices() on arguments

#include "argparse.hpp"

int main(int argc, char* argv[]) {
  argparse::ArgumentParser program("test");

  program.add_argument("input")
    .default_value("baz")
    .choices("foo", "bar", "baz");

  try {
    program.parse_args(argc, argv);
  }
  catch (const std::exception& err) {
    std::cerr << err.what() << std::endl;
    std::cerr << program;
    std::exit(1);
  }

  auto input = program.get("input");
  std::cout << input << std::endl;
}

foo@bar:/home/dev/$ ./main fex
Invalid argument "fex" - allowed options: {foo, bar, baz}

[github-actions]

⚠️ clang-tidy found issue(s) with the introduced code (1/1)