Added benchmark for the CSS parser.

benchmark/css/parser/parser_bench.rb

@@ -0,0 +1,9 @@
+require_relative '../../benchmark_helper'
+
+css = 'foo bar bar.some_class element#with_id[title="Foo"]'
+
+Benchmark.ips do |bench|
+  bench.report 'CSS' do
+    Oga::CSS::Parser.new(css).parse
+  end
+end