Comment on PR when config is invalid

* Hound users currently receive no feedback when their Ruby, JavaScript,
  and CoffeeScript configuration files are invalid. This commit
  introduces a new feature to address this disconnect:
  When a config file is empty or can't be parsed, Hound will leave
  a comment on your pull request indicating that the config file is
  invalid.
* Validate repo config in `BuildRunner#run`
* Add `RepoConfig#validate` and `RepoConfig#errors`
* Add `GithubApi#add_issue_comment` and `PullRequest#add_comment`
* https://trello.com/c/JLt3FVUb/397-comment-on-the-pr-that-breaks-configuration
Seems like this can just be `map`:
```rb
invalid_style_guides.map do |style_guide|
  I18n.t("invalid_config", config_file_name: config_path_for(style_guide))
end
```