Verify EthPM registry contract on deploy

[vbaranov]

In order to increase trust to the registry contract, it would be awesome if it automatically verified with ethpm registry deploy ... comand in blockchain explorers like Blockscout or Etherscan since both of them support verify API endpoints:

Blockscout: https://blockscout.com/eth/mainnet/api_docs (no need API key)
Etherscan: https://etherscan.io/apis#contracts (need API key)

[njgheorghita]

Great idea, thanks @vbaranov