If supplied, attach filename property to comment node loc. (#80)

src/parser/comments.js

@@ -33,6 +33,7 @@ function last(stack) {
 const pp = Parser.prototype;
 
 pp.addComment = function (comment) {
+  if (this.filename) comment.loc.filename = this.filename;
   this.state.trailingComments.push(comment);
   this.state.leadingComments.push(comment);
 };