Add a linter rule for newlines after PHP open tags
Summary: <?php\n\n... is much easier to read than <?php\n.... Depends on D13889 and D13890.
Test Plan: Added unit tests.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D13534