support native segwit regtest addresses (#148)

[cc-unchained]

Add support for Native Segwit addresses on regtest networks

Resolves Issue #148

To verify this human readable part for bech32 regtest addresses, the following references may be helpful:

• Bitcoin Core RegTestParams
• BIP-0173
• Implementation in bitcoinjs-lib

[Shadouts]

I think this is fuzzy. I think a root-level branch specific for regtest would be better because a bc1q with length 64 should throw.

...
elif s[:4] in ("bc1q", "tb1q"):
        if len(s) == 42:
            # p2wpkh
            return P2WPKHScriptPubKey(decode_bech32(s)[2])
        elif len(s) == 62:
            # p2wsh
            return P2WSHScriptPubKey(decode_bech32(s)[2])
elif s[:6] == "bcrt1q":
        if len(s) == 44:
            # p2wpkh
            return P2WPKHScriptPubKey(decode_bech32(s)[2])
        elif len(s) == 64:
            # p2wsh
            return P2WSHScriptPubKey(decode_bech32(s)[2])
...

[jimmysong]

Are there plans to add regtest as a separate network parameter? There are a lot of functions that use network as a parameter and if we're to support regtest, they all need to be updated. For example bech32.py has a function encode_bech32_checksum. That would need to return the address of the type that is in this function.