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.

先找出現有的 MutuallyExclusiveSet 和 HelpWriter 實作，然後檢視已解析的選項與驗證失敗如何流經剖析器。將要求的相依選用引數與提議的 CustomValidation 函式進行比較；完成的定義是實作一種已達成共識的行為，並以測試涵蓋其說明／錯誤處理。

由索引模型根據 Issue 內容生成。