Modernize ArcanistJSHintLinter.

Summary: Modernize ArcanistJSHintLinter by extending from ArcanistExternalLinter instead of ArcanistLinter.

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/D8965