Add :only option for limiting trigram search fields

Addresses:
https://groups.google.com/forum/#!topic/casecommons-dev/yOojbQoVdWE

I wasn't sure how active things were on the mailing list these days so I just went ahead and implemented a pull request. Please offer suggestions if this is not acceptable as is. 
That is wise. Are you saying that is just a good habit in general because I am not sure how it applies here since that variable input is controlled by the development code and not the query params or am I misunderstanding something?