Add default export for es format.

This is a minor addition that add's a default export statement (e.g. `export default { ... };`) for the es output format.

Fixes #525

lib/compiler/passes/generate-js.js

@@ -1315,6 +1315,8 @@ function generateJS(ast, options) {
           toplevelCode,
           "",
           "export " + generateParserExports() + ";",
+          "",
+          "export default " + generateParserObject() + ";",
           ""
         );