Make bin/lipsum generate hanldle generator keys and arguments more clearly

Summary:
Ref T12319. Currently, bin/lipsum uses substring matches against human-readable text to chose which objects to generate.

Instead:

• Use separate selector keys which are guaranteed to be unique.
• When a match is exact, select only that generator.
• When a match is ambiguous, fail and warn the user.

Test Plan: Generated several types of objects, tried to generate ambiguous objects like "e".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12319

Differential Revision: https://secure.phabricator.com/D17420