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
How about pulling lines 94 and 95 into an `invalid_style_guides` private method?

```ruby
def validate_style_guide_configs
  invalid_style_guides.each do |style_guide|
    error_message = I18n.t(
      "invalid_config",
      config_file_name: config_path_for(style_guide)
    )
    add_error(error_message)
  end
end

def invalid_style_guides
  STYLE_GUIDES.select do |style_guide|
    enabled_for?(style_guide) && self.for(style_guide).empty?
  end
end
```