Notify commit author directly, when applicable

[Cryptophobia]

From @vdice on October 5, 2016 17:29

It might be handy to send Slack notifications directly to the commit author, when applicable. This is easily done now that we've switched to using Slack's incoming webhook api (deis/jenkins-jobs#243) by setting channel as @user.

We currently derive the commit author email from the git commit, so, as long as a given author consistently uses the same email, we'd just maintain a userEmail -> slackHandle map and pull from this where relevant.

Copied from original issue: deis/jenkins-jobs#251