Add shallow cloning capabilities to Tokens to preserve the AST after rendering

lib/renderer.js

@@ -29,7 +29,10 @@ default_rules.code_block = function (tokens, idx /*, options, env */) {
 
 
 default_rules.fence = function (tokens, idx, options, env, slf) {
-  var token = tokens[idx],
+  // We will make modifications of the attributes object of this
+  // token while processing, so we obtain a shallow clone as to
+  // prevent modifying the token in the token list.
+  var token = tokens[idx].clone(),
       info = token.info ? unescapeAll(token.info).trim() : '',
       langName = '',
       highlighted;

lib/token.js

@@ -194,4 +194,28 @@ Token.prototype.attrJoin = function attrJoin(name, value) {
 };
 
 
+/**
+ * Token.clone()
+ *
+ * Obtain a shallow clone of the token.  You can use this while rendering to
+ * prevent modifying the token list while rendering.
+ */
+
+Token.prototype.clone = function clone() {
+  var token = new Token(this.type, this.tag, this.nesting);
+
+  token.attrs = this.attrs;
+  token.level = this.level;
+  token.children = this.children;
+  token.content = this.content;
+  token.map = this.map;
+  token.markup = this.markup;
+  token.info = this.info;
+  token.meta = this.meta;
+  token.block = this.block;
+  token.hidden = this.hidden;
+
+  return token;
+};
+
 module.exports = Token;