Better support for adding third-party contracts to deployments

[anders-torbjornsen]

Playmint have a use-case where the front-end uses the Zem deployment json file, so for mainnet deployments it needs to contain the address of the loot contract. Currently this needs to be added by hand, so a way to do this with code is needed, whereby a deploy script can tell a Deployment to include a certain address in there but never try to deploy the contract itself.