Add Goerli Support

[holgerd77]

This works syncing on the final version of the Goerli testnet:

./bin/cli.js --network=goerli
INFO [02-07|13:59:14] Data directory: /Users/hdrewes/Library/Ethereum/goerli/ethereumjs/chaindata
INFO [02-07|13:59:14] Initializing Ethereumjs client...
INFO [02-07|13:59:14] Fast sync mode
INFO [02-07|13:59:14] Connecting to network: goerli
INFO [02-07|13:59:14] Latest local block: number=0 td=1 hash=bf7e331f...
INFO [02-07|13:59:14] Synchronizing blockchain...
INFO [02-07|13:59:14] Started rlpx server.
INFO [02-07|13:59:14] Started libp2p server.
INFO [02-07|13:59:14] Listener up transport=rlpx url=enode://b323435b016cb8e10d53c70890daf09621b9cc2207ec4b861d2c60b9287cb1a4d45768ba243b01422bc847b0259a250d2d1a1a41845df06d7cbf7ae4a9246a63@[::]:30303
INFO [02-07|13:59:15] Listener up transport=libp2p url=/ip4/127.0.0.1/tcp/50580/ws/ipfs/QmZJuoW98KjdpxWpPZTtMUaGQdoRJ3rmgCdAhEsipZky3Y
INFO [02-07|13:59:15] Started eth service.
INFO [02-07|13:59:46] Imported blocks count=128 number=1 hash=8f5bab21... peers=1
INFO [02-07|13:59:48] Imported blocks count=256 number=129 hash=831d8538... peers=1
INFO [02-07|13:59:50] Imported blocks count=512 number=385 hash=e849bd62... peers=1
INFO [02-07|13:59:54] Imported blocks count=1024 number=897 hash=2da235c0... peers=1
INFO [02-07|13:59:58] Imported blocks count=1024 number=1921 hash=2bb6b20c... peers=1
INFO [02-07|13:59:58] Imported blocks count=128 number=2945 hash=abcc6f40... peers=1
INFO [02-07|14:00:05] Imported blocks count=1920 number=3073 hash=d8aa8483... peers=1
INFO [02-07|14:00:05] Imported blocks count=128 number=4993 hash=299148c0... peers=1

Would be cool to have this integrated, since on https://github.com/goerli/testnet they are still referring to the preliminary test version.

[coveralls]

Coverage remained the same at 92.731% when pulling 6bd7d03 on add-goerli-support into 1905b14 on master.

[vpulim]

Looks good! Thanks Holger