Add rake tasks for Rubocop

To always run rubocop with the same options (specially important when
regenerating the todo file).

It creates the following three tasks:

rake rubocop                     # Run RuboCop on the whole project
rake rubocop:auto_correct        # Auto-correct RuboCop offenses
rake rubocop:auto_gen_config     # Generate .rubocop_todo.yml

lib/tasks/rubocop.rake

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+require 'rubocop/rake_task'
+
+namespace :rubocop do
+  desc 'Generate .rubocop_todo.yml'
+  task :auto_gen_config do
+    sh 'rubocop --display-cop-names --auto-gen-config --auto-gen-only-exclude'
+  end
+end
+
+desc 'Run RuboCop on the whole project'
+RuboCop::RakeTask.new(:rubocop) do |task|
+  task.patterns = []
+  task.options = ['--display-cop-names']
+end