Update style guide link in README
Add config option to display cop names in comments

* add `ShowCopNames` config option to show cop names in GitHub comments
  * turned off by default
  * to enable set `ShowCopNames: true` in `.hound.yml`
Fix erroneous logic to disallow duplicate comments

* This commit fixes two logic errors in be2dd34178
* In `Commenter#previous_line_comments`, compare position of existing
  comment to line patch position rather than line number
* In `CommentPolicy#violation_not_previously_reported?`, compare new
  violation message to body content of existing comments rather than the comment
  objects
* https://trello.com/c/ha2hgFj2/283-avoid-commenting-a-second-time-after-a-force-push
Upgrade RuboCop

* memoize StyleGuide#config result
* add tests for old-style syntax
* call `add_missing_namespaces` on override (custom) config to ensure that
  merging is handled properly

https://trello.com/c/K7dSqe3k
Disable EndOfLine cop in tests

This cop breaks when dealing with strings (instead of files).

Here's a RuboCop PR to address the issue:
https://github.com/bbatsov/rubocop/pull/1203
Compare violation messages before commenting

* This commit ensures that:
  - to determine whether a comment is permitted, we check existing comments'
    `original_position` rather than `position`, which is nil after force pushing
  - violation messages in existing comments are split and checked
    individually, rather than as one large string with line breaks