Add rbtrace and stackprof and a script/benchmark script.

Gemfile

@@ -1,2 +1,7 @@
 source 'https://rubygems.org'
 gemspec
+
+if ENV['BENCHMARK']
+  gem 'rbtrace'
+  gem 'stackprof'
+end

script/benchmark

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+set -e
+
+export BENCHMARK=true
+script/bootstrap
+
+TEST_SCRIPT="Jekyll::Commands::Build.process({'source' => 'site'})"
+PROF_OUTPUT_FILE=tmp/stackprof-$(date +%Y%m%d).dump
+
+bundle exec ruby -r./lib/jekyll -rstackprof -e "StackProf.run(mode: :cpu, out: '${PROF_OUTPUT_FILE}') { ${TEST_SCRIPT} }"
+
+bundle exec stackprof $PROF_OUTPUT_FILE $@