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