add a bech32 prefix for bitcoin gold (#5)
add a bech32 prefix for bitcoin gold (#5)

`btg` is the prefix defined by [slip-0173](https://github.com/satoshilabs/slips/blob/master/slip-0173.md) and used in [the official bitcoinjs-lib fork](https://github.com/BTCGPU/bitcoinjs-lib/blob/master/src/networks.js#L7)
BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS