[feature] Add API to post a PR comment

[vmarkovtsev]

As we have already seen many times with running style-analyzer in practice, it is often confusing to people why no comments are generated. There can be several reasons, and some of them are warnings, e.g.: too small repo or no supported languages or too much time taken for training.

I suggest to add an API to post a regular message into the PR thread. We would use it to report those notices/warnings/errors. My favorite use case would be posting "I am still alive" messages every hour.

[vmarkovtsev]

cc @marnovo

[vmarkovtsev]

Example: https://src-d.slack.com/archives/C7USX021L/p1551791344036100 (private)

[smacker]

If you return comment without file as it's described in proto file, such comment will be posted as a normal regular comment. Does it solve your problem?

[dpordomingo]

to post regular messages, the way that @smacker suggested should be the ok.
But for other purposes Lookout could increase its api;
example: isMergeable, as part of the Analyzer EventResponse, could be used for Lookout to mark the PR as mergeable or not (as Travis does).
... and many other ways to communicate common things from the Analyzer