Modularize Ferret fulltext functions

Summary: Ref T13511. Currently, Ferret fulltext field functions (like "title:") are hard-coded. Modularize them so extensions may define new ones.

Test Plan: Added a new custom field which emits data for the indexer, searched for "animal-noises:moo", "animal-noises:-", etc., in global search and application search.

Maniphest Tasks: T13511

Differential Revision: https://secure.phabricator.com/D21131