Slightly loosen the mention regexp to allow for "Go ask @tomo."

Summary: Currently, we incorrectly flag these under the "@lists.com" rule.
Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-887c2bc66eb9d47d02f4/
Reviewed By: tomo
Reviewers: tomo
CC: aran, epriestley, tomo
Differential Revision: 708