[Fix #1435] Add a new cop for PluckOnSelect.

[blnoonan]

Select returns an ActiveRecord relation with only the selected column(s) marked for retrieval. Pluck returns an array of column values. Using them together is at best redundant and at worst confusing and inefficient. When chained with .select, .pluck is unaware of any RAW SQL directive passed to .select (e.g. column aliases or a DISTINCT clause). This can lead to unexpected behavior.

The aggressive style of this cop asserts that there is always a better alternative than using pluck and select together. A conservative style is offered for people who prefer to ignore this cop for selects with block arguments.

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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.