Add ability to add "extra" info to smart contract manifest

[spencercorwin]

Every manifest has the ability to include a property called "extra" which is an arbitrary user-defined JSON object. We should probably just add the ability to add an optional "extra" property to a smart contract which will be parsed by a simple JSON parser and included in the manifest. Might need to check for malicious code or something too? See ManifestSmartContractProcessor.ts