Allow IndentationWidth cop to ignore certain lines from indentation

See: bbatsov/rubocop#1031
You should also check that `body_node.children.first.type` is `:send` before extracting the method name. These child nodes can be something other than method calls, and then we should not try to get the name.

And use `_receiver, method, *_args = *body_node.children.first` for better clarity (and looking more like the same thing in other parts of the code).