Let have the bot explain why it doesn't merge a PR

[bnjbvr]

Checklist

• [x] I have read the Contributor Guide
• [x] I have read and agree to the Code of Conduct
• [x] I have added a description of my changes and why I'd like them included in the section below

Description of Changes

This makes it possible to ping the bot in a PR comment (not a review comment; not a response to a review comment) and have the bot answer with the current merge status, and list the reasons why it's not merged a PR yet.

See it in action over here: https://github.com/bnjbvr/octotest/pull/9

Related Issues

Fixes #32.