Add profile:stackprof rake task.

performance/stackprof.rb

@@ -0,0 +1,15 @@
+require 'stackprof' rescue fail("install stackprof extension/gem")
+require File.dirname(__FILE__) + '/theme_runner'
+
+profiler = ThemeRunner.new
+profiler.run
+results = StackProf.run(mode: :cpu, out: ENV['FILENAME']) do
+  100.times do
+    profiler.run
+  end
+end
+if results.kind_of?(File)
+  puts "wrote stackprof dump to #{results.path}"
+else
+  StackProf::Report.new(results).print_text(false, 20)
+end