[Fix #7705] Fix Style/OneLineConditional to handle if/then/elsif/then/else and add AlwaysCorrectToMultiline config option

[dmytro-savochkin]

Fixes #7705.

This is continuation of work started in #7710 by https://github.com/Lykos. I basically used the code proposed in that PR, fixed some minor things, added more specs, and also added a new config parameter called AlwaysCorrectToMultiline (I think this addresses the suggestion raised in #7710 (comment)).

---

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]

Thanks for driving this to the finish line!