Add support for Dogecoin regtest

[edxeth]

https://github.com/spesmilo/electrumx/blob/master/electrumx/lib/coins.py includes Dogecoin(AuxPowMixin, Coin) and DogecoinTestnet(Dogecoin) but it lacks DogecoinRegtest(DogecoinTestnet) for some weird reason.

class DogecoinRegtest(DogecoinTestnet):
    NAME = "Dogecoin"
    SHORTNAME = "XDG"
    NET = "regtest"
    GENESIS_HASH = ('3d2160a3b5dc4a9d62e7e66a295f7031'
                    '3ac808440ef7400d6c0772171ce973a5')
    PEERS = []
    TX_COUNT = 1
    TX_COUNT_HEIGHT = 1