Add new Style/HeredocDelimiters cop

[Drenmi]

This change adds a cop for enforcing meaningful heredoc delimiters. It is based on a delimiter blacklist, which by default contains END.

RuboCop had 3,207 offenses, so I separated the cop implementation and fixing of existing offenses into two different commits. 😉

⚠️ Opening the commit which fixes the offenses in GitHub might freeze your browser.

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Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Used the same coding conventions as the rest of the project.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[bbatsov]

One offense in the new cop is failing the build.

[Drenmi]

@bbatsov: Fixed! There was also a new END merged into master after the PR was opened, which caused some serious confusion as to why the build was failing. 😅

[bbatsov]

Awesome work, btw!