optionArgumentSeparator should allow multiple values

[joffrey-bion]

Some CLIs don't enforce one convention, but accept several. The Clikt library accepts both spaces and = as separators between option names and values (--foo bar and --foo=bar).

It would be nice to allow expressing this in the spec.

[joffrey-bion]

Note: there could be a preferred option argument separator, the rest being just 'acceptable'. This could be expressed as a separate field or just with the order of the list. Probably declaring that the first separator is the preferred approach might be enough.

[devin-fisher]

I've found this need too. I would put from my experience that there is not really a preferred option so much as a more commonly used convention. I think the order in an array would be adequate to communicate this. I've never seen a tool that really works better with one over another.