Allow gulped modules to extend standard SObjects with additional standard fields

The standard SObject definitions include most fields but there are some features, such as person Accounts, which add additional custom fields that it is difficult for us to capture. To address this I propose that we allow standard objects to be extended with additional standard fields in the gulp metadata format.

At the moment standard fields are removed from SObject metadata within SObjectGenerator. If we remove this filtering we can add module aware field filtering into SObjectDeployer that is more flexible.