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. 
Usually I try to normalize on string keys, especially as interning a user-supplied value can result in a DoS (by filling memory with symbols).