Add configuration option to Rails::HelperInstanceVariable to allow instance variables as default method arguments

[rmm5t]

Is your feature request related to a problem? Please describe.

I generally agree with the Rails::HelperInstanceVariable cop, but I believe there are reasonable scenarios where an instance variable can (and perhaps should) be used as a default argument to a helper method.

Here's a contrived example:

  def widget_title(widget = @widget)
    "#{widget.name} (#{widget.size})"
  end

Describe the solution you'd like

I'd like to be able to keep the Rails::HelperInstanceVariable cop in play for instance variables within the helper method body, but allow instance variables as default arguments.

Rails/HelperInstanceVariable:
  AllowArguments: true               # or something similar

Is this something that rubocop-rails team might consider if a PR was submitted?