Support for SLIP132's custom BIP32 prefixes

At least trezor and electrum support [SLIP132 base58 prefixes](https://github.com/satoshilabs/slips/blob/master/slip-0132.md) for extended keys, which indicate the script type to be derived.

I think it could be worth adding a flags parameter to bip32_key_from_base58[,_alloc], otherwise could expose the behavior through a separate function and avoid the BC break?