Ordered vector of group names to preserve order of groups

[nigels-com]

Thanks for cxxopts, it's fits nicely with our needs.

I experimented with the grouping support today. I noticed that the order of the groups isn't insertion order - it's based on the std::map comparator. So I felt that we could add a std::vector of the group names here in the order of insertion. Two benefits:

• The order of groups in the C++ code (insertion order) will match the -h output.
• The order of groups in the output will be deterministic and predictable, even across platforms and toolchains.

For consideration.

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This change is 

[jarro2783]

Thanks, looks good.