Add Blocknet (BLOCK) support

[rikublock]

Add Blocknet support to Blockbook.

Coin Information:

• Website: https://blocknet.co/
• Docs: https://docs.blocknet.co/
• GitHub: https://github.com/blocknetdx
• Coin Market Cap: https://coinmarketcap.com/currencies/blocknet

Projects running on Blocknet:

• BlockDX: https://blockdx.net/ (decentralized exchange)
• XQuery: https://xquery.io/ (decentralized blockchain data indexing)