Return fulltext tokens from the Ferret fulltext engine

Summary:
Ref T12819. These render the little "Searched For: X, Y, U V" hint about how something was parsed.

(This might get a "substring" color or "title only" color or something in the future.)

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18589