nconf: Warning: Use Cipheriv for counter mode of aes-256-ctr

Starting in at least node v8.7.0, this warning is emitted when using the default algorithm. The recommended approach is to use an iv and call crypto.createCipheriv(algorithm, key, iv) instead of crypto.createCipher(algorithm, key). I’m guessing this would change the API of secure-keys to require an iv in addition to a secret in the options to instantiate it.