Check for unrecognised YAML keys

[Ortham]

Unrecognised YAML keys are allowed at the top level of a metadata file, but below that the values of recognised keys should be the data structures defined for metadata files, and the presence of keys other than those defined for a data structure may indicate a typo. @sibir-ine has requested that the validator warn about such keys.

The YAML parsing isn't exposed to the metadata validator, and I don't think warnings would be useful in LOOT so doing the checks in libloot wouldn't be appropriate. We used to have a schema file that we validated masterlists against, and I think that validation warned about unrecognised keys, so it might be better to reintroduce that kind of validation as a separate step. Alternatively, the metadata validator could do a separate parse of the metadata file that checks for unrecognised keys, but this would involve pulling in yaml-cpp as a duplicate dependency.