Express interaction between options (mutually exclusive, co-occurring, etc.) #65

I use the Clikt library for all my Kotlin CLI tools. One of the huge perks of this CLI library is that it allows to express in detail what is and isn't valid, providing very precise validation and error messages, so we can focus on the actual commands implementation.

It would be nice if the OpenCLI spec could also express this in more details. In particular, provide a way to express interaction between options: mutually exclusive options, co-occurring option groups, group switches, etc.