Added CSS/XPath Parser.parse_with_cache

This method parses and caches ASTs using Oga::LRU. Currently the default
of 1024 keys is used.

See #71 for more information.

benchmark/css/parser/parser_bench.rb

@@ -3,7 +3,13 @@ require_relative '../../benchmark_helper'
 css = 'foo bar bar.some_class element#with_id[title="Foo"]'
 
 Benchmark.ips do |bench|
-  bench.report 'CSS' do
+  bench.report 'without cache' do
     Oga::CSS::Parser.new(css).parse
   end
+
+  bench.report 'with cache' do
+    Oga::CSS::Parser.parse_with_cache(css)
+  end
+
+  bench.compare!
 end