Add source line number attribute in order to allow sync scrolls

lib/renderer.js

@@ -219,6 +219,19 @@ Renderer.prototype.renderToken = function renderToken(tokens, idx, options) {
   // Add token name, e.g. `<img`
   result += (token.nesting === -1 ? '</' : '<') + token.tag;
 
+  // Add source line number attribute in order to allow sync scrolls
+  // Idea extracted from (https://markdown-it.github.io/index.js)
+  if (options.lineNumber) {
+    if (token.map && token.level === 0) {
+      var attr = [ 'data-line', token.map[0] ];
+      if (token.attrs) {
+        token.attrs.push(attr);
+      } else {
+        token.attrs = [ attr ];
+      }
+    }
+  }
+
   // Encode attributes, e.g. `<img src="foo"`
   result += this.renderAttrs(token);