load_config should check for names not present in config.default

[ressy]

Currently load_config will return whatever structure is in a given YAML file, but there's no error or warning message for unrecognized entries. All valid names should already be present in config.defaults (even if they just point to NULL) so this should be used as a simple check when loading configurations. This would easily catch YAML formatting problems like key:val versus key: val since the former would show a mismatch with the default config.