Layout/ClassStructure cop should allow grouping macros by their visibility

[gPrado]

Expected behavior

There should be a way to configure different visibility levels for macros to avoid false positives with the ordering of protected/private macros.

Actual behavior

Currently, there is no way to make such configuration.

Steps to reproduce the problem

Given the following configuration:

Layout/ClassStructure:
  Categories:
    - attribute:
      - attr_accessor
      - attr_writer
      - attr_reader
  ExpectedOrder:
    - attribute
    - initializer

And the following class definition:

class Person
  attr_accessor :name

  def initialize(name)
    @name = name
  end

  private

  attr_accessor :age
end

Rubocop gives the following output:

person.rb:10:3: C: Layout/ClassStructure: attribute is supposed to appear before initializer.
  attr_accessor :age
  ^^^^^^^^^^^^^^^^^^

RuboCop version

$ rubocop -V
0.52.1 (using Parser 2.4.0.2, running on ruby 2.3.0 x86_64-darwin16)