Add MinSize option to Style/SymbolArray

[scottmatthewman]

In line with Style/WordArray, allow a MinSize setting to determine the number of items to allow per array before the cop kicks in.

The minimum size handling is extracted into a new mixin, and some other similar functionality between the two cops has been better reflected with some method renaming and reordering.

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• 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).

[scottmatthewman]

I've only included the generate_cops_documentation for this cop in the PR – there are a few outstanding changes in master that are yet to be reflected in the docs but it'd be inappropriate to include them here.

[bbatsov]

I'd extend this documentation a bit, perhaps referencing the use-cases warranting the existence of the mixin. At the very least array count minimums sounds pretty vague and confusing to me.

[scottmatthewman]

That's a fair point. I've expanded the documentation on the mixin to specify which configuration option it is handling, and then expanded the doc comments on both Style/SymbolArray and Style/WordArray to explain the configuration parameter.