Add subexpression benchmark

bench/templates/subexpression.js

@@ -0,0 +1,14 @@
+module.exports = {
+  helpers: {
+    echo: function(value) {
+      return 'foo ' + value;
+    },
+    header: function() {
+      return "Colors";
+    }
+  },
+  handlebars: "{{echo (header)}}",
+  eco: "<%= @echo(@header()) %>"
+};
+
+module.exports.context = module.exports.helpers;