Config object: Warn for unused keys
Keys that have been defined but never used can be problematic since this might come from a typo that the user might never be aware of. A way to address this issue would be to keep a map of bools for each keys, getting set to true when they have been read (using a getValue), so it would always be possible for the calling application to know how many keys have never been used (and potentially to get the list of such keys).