Reference implementation for BIP85

Includes a CLI tool to test the functionality. Example usage (requires installation beforehand):

bip85-cli --index 0 --xprv xprv9s21ZrQH143K2LBWUUQRFXhucrQqBpKdRRxNVq2zBqsx8HVqFk2uYo8kmbaLLHRdqtQpUm98uKfu3vca1LqdGhUtyoFnCNkfmXRyPXLjbKb bip39

See bip85-cli --help for all options.