Update wording so that it could work as warning or fatal error.
Update getConfigSources() to start returning empty objects nows.

Also, while a warning would previously be issued if only empty
configuration files were found, the warning will not be issued in that
case, only if the config files are warning.

This could be considered to address #160, since we no longer claim that
empty configuration file is missing.
Update Strict Mode to warn-by-default and upgrade warnings to exceptions under strict mode.

   Also strictness checks are now skipped for NODE_ENV=development because 'development' is the default value.