Add a linter rule for determining when single quotes should be used over double quotes.

Summary: Personally, I am a strong fan of this rule. There is currently a similar rule provided by PHP_CodeSniffer.

Test Plan: Wrote and executed unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9117