Add support for multi-contracts

[andreibancioiu]

Related to #31.

Now, building the multisig contract would result into the following artifacts.json:

{
    "buildMetadata": {
        ...
    },
    "buildOptions": {
        ...
        "specificContract": "multisig",
        "buildRootFolder": "/tmp/project"
    },
    "contracts": {
        "multisig-full": {
            "version": "0.0.0",
            "codehash": "6a739f31c52fd2284ff77a8ef6f5c50f94ab8511fe1f772e5400d3c0e425a46b",
            "artifacts": {
                "bytecode": "multisig-full.wasm",
                "abi": "multisig-full.abi.json",
                "srcPackage": "multisig-0.0.0.source.json"
            }
        },
        "multisig-view": {
            "version": "0.0.0",
            "codehash": "50da33a67b84a8b754ac8efdb30cb9424ff2173622a6d6083522c21f36bde8ac",
            "artifacts": {
                "bytecode": "multisig-view.wasm",
                "abi": "multisig-view.abi.json",
                "srcPackage": "multisig-0.0.0.source.json"
            }
        },
        "multisig": {
            "version": "0.0.0",
            "codehash": "5cc02a06d3c921a31ecbfe351a69ac2309c7c42a4a82152ee0df09cf8c4c73b6",
            "artifacts": {
                "bytecode": "multisig.wasm",
                "abi": "multisig.abi.json",
                "srcPackage": "multisig-0.0.0.source.json"
            }
        }
    }
}