Add default account option

[iamdefinitelyahuman]

Currently if you attempt to deploy a contract without explicitly declaring from, brownie raises AttributeError: Contract has no owner, you must supply a tx dict with a 'from' field as the last argument.

1. This error is confusing. It's meant to be given from an already deployed contract.
2. It should be possible to declare a default account, similar to web3.eth.defaultAccount, that contracts will be deployed from. Brownie already has the default_contract_owner config setting, so perhaps expand on that?