[Fix #5388] Add new Style/UnlessMultipleConditions cop

[wata727]

Fixes #5388

This cop checks that unless is not used with multiple conditions.
In general, using multiple conditions with unless reduces readability.

However, RuboCop itself doesn't use this cop because it currently supports Ruby 2.1 and 2.2.
In Ruby version not implementing Hash#dig, the following code is useful.

return unless hash[:a] && hash[:a][:b]

Considering the above example, should this cop be disabled by default?

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[bbatsov]

I've got one remark about naming - technically you always have a single condition expression, but it might be "composite"/"complex" when composed from several expressions.

I also wonder how this play with the existing cop suggesting the use of unless when possible. I guess it should be configurable to suggest the replacement only in non-composite scenarios.

[wata727]

I've got one remark about naming - technically you always have a single condition expression, but it might be "composite"/"complex" when composed from several expressions.

Maybe, that would be right. However, if the conditional expression is single, In the "style" it is not a problem.
If this name was something like Style/UnlessComplexCondtions, that would be a problem.

I also wonder how this play with the existing cop suggesting the use of unless when possible. I guess it should be configurable to suggest the replacement only in non-composite scenarios.

Umm, this is a very difficult problem...
The usage of unless or if should be selected by a user according to a meaning of conditions, and I guess that RuboCop cannot propose the usage mechanically.

I have to think more about these problems. So, I will close this pull request once. Thank you for your opinion!