Period/dot in username breaks mention syntax

As reported by @NightmareJoker2, https://gitter.im/gitterHQ/gitter?at=5b1be1b0e36e0e7d3f60c2a2

A GitLab usernames can have a . period/dot in them which breaks the mention syntax

@rafael.espindola_gitlab -> In this case pings @rafael

Relevant code

• https://gitlab.com/gitlab-org/gitter/gitter-marked/blob/87b23ce87dbe9b2f7695551bf6c631d862beed3a/lib/marked.js#L486