Feature request: built-int choices arguments support.

[KindRoach]

To restrict the set of values for an argument, a custom action is needed as the example in README.md:

program.add_argument("input")
  .default_value(std::string{"baz"})
  .action([](const std::string& value) {
    static const std::vector<std::string> choices = { "foo", "bar", "baz" };
    if (std::find(choices.begin(), choices.end(), value) != choices.end()) {
      return value;
    }
    return std::string{ "baz" };
  });

Built-int choices arguments support will be more convenient, such like:

program.add_argument("input")
  .default_value(std::string{"baz"})
  .choices("foo", "bar", "baz");

Thanks!

[TheFGFSEagle]

Is this going to be implemented any time soon ? Can I be of any help ?

[TheFGFSEagle]

Thanks ! :)