[Fix #1075] Add new Rails/SelectMap cop

[koic]

Resolves #1075.

This PR adds new Rails/SelectMap cop that checks for uses of select(:column_name) with map(&:column_name). These can be replaced with pluck(:column_name).
There also should be some performance improvement since it skips instantiating the model class for matches.

# bad
Model.select(:column_name).map(&:column_name)

# good
Model.pluck(:column_name)

---

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.

[vlad-pisanov]

This should be marked unsafe, since the model might overload the attribute getter; also the model's after_initialize hooks are skipped when pluck is used.

[Earlopain]

Cool, thank you!