[Fix #368] Add DelegatePrivate cop

[povilasjurcys]

This PR solves #368 issue by adding DelegatePrivate cop. This cop ensures that private: true is used when delegating methods in private is skipped in public scope. Here are more cases:

# bad
class Foo
  def bar=Bar.new

  private

  delegate :baz, to: :bar
end

# bad
class Foo
  delegate :baz, to: :bar, private: true

  def bar=Bar.new
end

# good
class Foo
  def bar=Bar.new

  private

  delegate :baz, to: :bar, private: true
end

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.