Add basic support for ENS

[iamdefinitelyahuman]

The Issue

Brownie currently does not support the Ethereum Name Service.

Implementation

web3py already supports ENS, so all we have to do on this side is remove the check for a valid hexstring before passing along addresses. If a given address ends with .eth allow it with no checks, otherwise proceed as normal.

One issue is that attempts to resolve an ENS address via web3py raises an InvalidAddress if not connected to the main net. In situations where the user is using a different network we should connect to the main net via a temporary web3 instance, resolve the name, and disconnect. This will require a mainnet host declared in config.py, which means defining a standard name for that network. A similar behaviour will be needed for #175 - these two issues will likely end up handled together.

We should also be caching the resolved addresses on a per-session basis to avoid repetitive lookups.