Add AllowIfModifier option to Style/IfInsideElse cop

[koic]

This PR adds AllowIfModifier option to Style/IfInsideElse cop.

This new option is based on rails/rails repo use case shown at the following URL.
rails/rails#36495.

This option works as follows.

AllowIfModifier: false (default)

AllowIfModifier is false by default. So breaking change will not happen.

# bad
if condition_a
  action_a
else
  action_b if condition_b
end

AllowIfModifier: true

It allows the following case when AllowIfModifier is true.

# good
if condition_a
  action_a
else
  action_b if condition_b
end

---

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.
• 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.

[bbatsov]

The proposed change is fine by me. Thanks!