Support for arguments that require each other but are optional

Similar to the MutuallyExclusiveSet, it would be very helpful to be able to specify that if argument A is specified then argument B is required, but if A is not specified, then B is optional.

This may be a very specific use case, and I wouldn't even pursue it, but I'm having difficulty using the built in HelpWriter when my additional validation fails, since this validation is outside of the normal flow.

An alternative would be to support passing a Function to something like parser.CustomValidation() where the developer could get the parsed options (that were properly parsed) and run additional validation checks. If a check fails the developer can raise an exception.

まず既存の MutuallyExclusiveSet と HelpWriter の実装を見つけ、次に解析されたオプションと検証失敗がパーサーを通じてどのように処理されるかを確認します。要求されている依存するオプション引数と、提案されている CustomValidation 関数を比較します。合意した1つの動作が実装され、そのヘルプ／エラー処理がテストでカバーされれば完了です。

索引モデルが issue の本文から書いたものです。