Support BIP39 mnemonic seed output for private keys

[bernardladenthin]

See

• https://github.com/bitcoin/bips/blob/master/bip-0039/bip-0039-wordlists.md
• https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/org/bitcoinj/crypto/MnemonicCode.java