Add new Rails/Pluck cop

[eugeneius]

pluck can be used instead of map to extract a single key from each element in an enumerable. When called on an Active Record relation, it results in a more efficient query that only selects the necessary key.

The pluck method was available in Active Record before Rails 5.0, but it wasn't implemented on Enumerable until that version, which is what allows this cop to auto-correct safely.

I've opened a pull request to add entries about pluck and pick to the style guide: rubocop/rails-style-guide#263.

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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).
• 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.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

This looks useful 👍