Support peppering hashes

[dhui]

• Add PepperGrinder interface with methods:
  • Encrypt(dest, plaintext []byte) ([]byte, error)
  • Decrypt(dest, ciphertext []byte) ([]byte, error)
• Add PepperGrinderAesGcm struct with fields:
  • Key []byte
  • nonce []byte - e.g. IV
• Add Peppers []PepperGrinder field to Config struct
• Add PepperVersion int field to Credential struct - index to Config.Peppers
• Update pepper keys when updating hashes