Add a rule to the XHPAST linter for detecting elseif usage.

Summary: This rule is adapted from PHP_CodeSniffer and is used to enforce the consistent use of else if.

Test Plan: Wrote and executed unit tests.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin, neal

Differential Revision: https://secure.phabricator.com/D9193