Cleanup specs

* use described_class
* do not use instance variables
* do not start example description with should
[Fixes #1094] Fix bug in ClassAndModuleChildren

ClassAndModuleChildren incorrectly flaged class with a single method
Merge pull request #1095 from geniou/rspec_cleanup

Cleanup specs
Merge pull request #1096 from geniou/class_and_module_children_single_method

[Fixes #1094] Fix bug in ClassAndModuleChildren
Fix incorrect error message about rubocop-todo.yml
Merge pull request #1098 from agrimm/todo_filename

Fix incorrect error message about rubocop-todo.yml
Add -F/--fail-fast option

Inspect files in order of modification time, newest first. And stop after the first file with offenses.
Merge pull request #1089 from jonas054/fail_fast

Add -F/--fail-fast option
Cut 0.22.0
[Fix #1104] Fix EachWithObject with modifier if
Merge pull request #1105 from geniou/each_with_object_fix

Fix EachWithObject with modifier if
Add optional require directive to .rubocop.yml
Merge pull request #1102 from geniou/config_require

Add optional require directive to .rubocop.yml
Add section about custom cops [ci skip]