add default empty env object

lib/index.js

@@ -488,6 +488,7 @@ MarkdownIt.prototype.use = function (plugin /*, params, ... */) {
  * and then pass updated object to renderer.
  **/
 MarkdownIt.prototype.parse = function (src, env) {
+  env = (env) ? env : {}
   var state = new this.core.State(src, this, env);
 
   this.core.process(state);