Support "-term" in Ferret engine queries

Summary:
Ref T12819. Supports negating search terms, e.g. "apple -honeycrisp".

When negating a term, we're a little more strict about what can match (that is, what can prevent a document from being returned) since it's easy for a user to type "apple -honeycrisp -honey -crisp -crispies -olcrispers -honeyyums" to keep refining their search, but hard/impossible to split apart an overboard term.

Test Plan:

• Ran apple -smith, apple -"granny smith", etc.
• Verified phone -tact does not exclude phone contact.
• (In theory, phone -~tact would, but the parser currently doesn't support this, and I'm not champing at the bit to add support.)

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18502