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. 
Yes, it was just a general comment. Though I'd still probably stringify here too - but that might just be personal preference/consistency.