Merge @default_config['groups'] with @people_config['groups']

@roderik

Thanks for fixing the `|` issue.

My original intention was to combine both the `default_config` groups if found with the `people_config` groups, but I realise now this would have caused failure if default_config['groups'] was not defined.

Fortunately this issue was spotted when you merged the original change in and Array.union was replaced with an || condition.

This pull request restores the behaviour I originally intended. 

Now might be a good time to introduce some unit tests. Happy to look into this.