Enhanced handling of fields option

[tstiemerling]

When the entity definition has a hidden: byDefault field, it is quite tedious to make internal calls to resolve the entity, since all the other fields have to be specified as well as the hidden one. It would be nice if fields option could be enhanced to support +/- modifier, which would mean either add or remove the given field from the default field list.

[icebob]

Good idea, could you create a PR?