Correctly identify `@todo` comments as TODOs.

Summary: Allow `@todo` comments to be linted as TODOs as well as `TODO` comments.

Test Plan: I added a new test case (`todo.lint-test`)

Reviewers: #blessed_reviewers, epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8389