Feature request: add a mechanism to allow people to ask enarxbot to add more reviewers

[connorkuehl]

If enarxbot can react to being mentioned in the comments of a PR someone could post something like:

@enarxbot bump

And enarxbot can assign an additional reviewer from the reviewer pool. This could be helpful for large or long running, or unanswered PRs as it will increase the number of eyes on it which will increase its likelihood of getting reviewed.