Add a linter rule for variable reference spacing

Summary: Adds a linter rule for spacing after the & token, similar to ArcanistUnaryPrefixExpressionSpacingXHPASTLinterRule.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14602