Add compatibility for anvil

[chalex-eth]

Foundry just released anvil, which aim to replace ganache and hardhat RPC. Foundry ecosystem is highly growing at speedlight, been able to run anvil in brownie console instead of ganache-cli would be awesome.

https://github.com/foundry-rs/foundry/tree/master/anvil

[pratraut]

Need to add support for forking with block number. I think we need to add fork_block to a list at https://github.com/eth-brownie/brownie/blob/master/brownie/_cli/networks.py#L48