Allow multiple namespace flags

[nmaggioni]

Since the [ignore-]checks flags can already be specified multiple times, now it can be done with the [ignore-]namespaces flags too. Their output is logically AND'ed.

The long options have been renamed from namespace to namespaces for consistency with the rest of the flags. This is a breaking change for those who use the long options but not the shorthands.

The precondition of specifying either includes or excludes is still valid.

[adamwg]

Thanks for the contribution!

This change itself looks good, but I would like to preserve backward compatibility with the existing flag names so we don't break anyone who might be using clusterlint from scripts/CI/etc.

[adamwg]

To preserve backward compatibility you can add namespace as another alias here. It will look a little awkward in the help, but I'd like to avoid breaking existing users. I.e.:

Name: "n, namespaces, namespace",

[nmaggioni]

Sure, makes sense. Do you want to add singular aliases to the checks and groups flags too (ex.: Name: "c, checks, check",) to show a common pattern?