Implement markdown-it token attrSet() method ourselves for compatibility with markdown-it>=4.0

Author: revin

index.js

@@ -9,13 +9,24 @@ module.exports = function(md, options) {
 for (var i = 2; i < tokens.length; i++) {
 if (isTodoItem(tokens, i)) {
 todoify(tokens[i], state.Token);
-tokens[i-2].attrSet('class', 'task-list-item');
-tokens[parentToken(tokens, i-2)].attrSet('class', 'task-list');
+attrSet(tokens[i-2], 'class', 'task-list-item');
+attrSet(tokens[parentToken(tokens, i-2)], 'class', 'task-list');
 }
 }
 });
 };
 
+function attrSet(token, name, value) {
+var index = token.attrIndex(name);
+var attr = [name, value];
+
+if (index < 0) {
+token.attrPush(attr);
+} else {
+token.attrs[index] = attr;
+}
+}
+
 function parentToken(tokens, index) {
 var targetLevel = tokens[index].level - 1;
 for (var i = index - 1; i >= 0; i--) {