update :spec rake task to allow example pattern as an argument

Rakefile.rb

@@ -13,7 +13,11 @@ require "rake/clean"
 CLEAN.include %w[build_test_run .yardoc doc coverage]
 CLOBBER.include %w[pkg]
 
-RSpec::Core::RakeTask.new(:spec)
+RSpec::Core::RakeTask.new(:spec, :example_string) do |task, args|
+  if args.example_string
+    task.rspec_opts = %W[-e "#{args.example_string}" -f documentation]
+  end
+end
 
 YARD::Rake::YardocTask.new do |yard|
   yard.files = ['lib/**/*.rb']