Comments on PR with success message if configured

Here's the problem: I am an expert programmer who makes 0 mistakes :trollface:. When I create a pull request without any style violations, I get stuck in 'limbo' waiting for comments from Hound. There is no way for me to know whether this is because I don't have Hound setup correctly, or because Hound is down, or because there were no violations.

My proposed solution is the ability for the user to add a special option to `.hound.yml`, like so:

~~~yml
SuccessMessage:
  Enabled: true
~~~

When present, Hound will comment with 'No style issues found! :+1:'.

I personally think that this behavior should be the default instead of opt-in, but I will leave that to the maintainers to decide.

Finally, this code base is a joy to work on! :heart: