Make colon in @mention optional
Make colon in @mention optional

It seems like Slack just change the behavior of tab-completing an
@mention to not include the colon.

Typing `@phab<TAB>` now results in `@phabulous ` instead of the previous
behavior of `@phabulous: `. For convenience and compatibility, support
both.

etcinit/phabulous,etcinit/phabricator-slack-feed,etcinit/phabulous