[feature request] add circleci webhook to pouchrobot

[allencloud]

Currently pouch project uses circleci to check the markdown file validation. If it fails, we need to tell users where to check the rule.

So if circleci fails, it should throw errors to webhook receiver(pouchrobot), and dear robot would attach a comment to the pull request.

When attaching a comment to pr, we should aggregate the error into a single one since travisCI would fail as well.