Add option to exclude systems

[massivebird]

We have the --systems option, but we could also implement an option called --exclude that analyzes all but the excluded systems. The two options would be made mutually exclusive.

Introducing such an option would probably necessitate renaming --systems to --only in order to establish consistent naming conventions. Just be CAREFUL about it >:|

Yeah, this idea seems a little pointless. I can't think of a realistic use case, but I'll post the issue anyway!