Setup code coverage with simplecov

Rakefile

@@ -10,3 +10,13 @@ RSpec::Core::RakeTask.new(:spec)
 RuboCop::RakeTask.new
 
 task default: %i[spec rubocop]
+
+# Delete these files and folders when running rake clobber.
+CLOBBER.include('coverage', '.rspec_status')
+
+desc 'Run spec with coverage'
+task :coverage do
+  ENV['COVERAGE'] = 'true'
+  Rake::Task['spec'].execute
+  `open coverage/index.html`
+end