Add a JSON linter.

Summary: Provide bindings for [[https://github.com/zaach/jsonlint | JSONLint]], which is a useful tool for linting and validating JSON. Theoretically, this could be done with pure PHP, however it would not be trivial (`json_decode`, for example, does not provide any context as to JSON validation errors).

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/D8988