Add an XML linter.
Summary: Add a linter which uses SimpleXML to detect errors and potential problems in XML files.
Test Plan: Wrote and executed unit tests.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D8989