Add --ignore option

[Reinmar]

Just like Lerna's --ignore https://github.com/lerna/lerna#--ignore-glob

We need it to skip ckeditor5-build-* when doing some maintenance tasks on CKEditor 5 packages.

We can also implement --scope so e.g. one can do:

--scope "package-*" --ignore "package-util-*

And I've been also thinking about "groups", defined in mgit.json so one can easily pick on which packages the command should be executed without writing the globs every time.

Anyway, let's start with --ignore for now and possibly --scope.