[Fix #204] Add Performance/Sum option to ignore potential false positives

[leoarnold]

Some codebases may contain non-numeric classes which implement a :+ method.
If a, b and c are objects of such a class, then certain corrections
do not apply:

[a, b].reduce(c, :+) # is equivalent to
[a, b].sum(c)

[a, b].map(&:to_i).sum # is equivalent to
[a, b].sum(&:to_i)

[a, b].reduce(:+) # works
[a, b].sum # raises TypeError

For users who wish to only register offenses where auto-correction
is unproblematic, we add the option OnlySumOrWithInitialValue
to the Performance/Sum cop.

The option is disabled by default, so standard behavior is preserved,
and can be activated by setting OnlySumOrWithInitialValue: true.

---

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 (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.
• 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]

Thanks!