allow ID of container node to optionally be provided in remark_config

frontend/app/common/config-types.ts

@@ -15,6 +15,7 @@ export interface CommentsConfig {
   max_shown_comments?: number;
   theme?: Theme;
   page_title?: string;
+  node?: string;
 }
 
 export interface LastCommentsConfig {

frontend/app/embed.ts

@@ -19,7 +19,7 @@ async function init(): Promise<void> {
   __webpack_public_path__ = HOST + '/web/';
   await loadPolyfills();
 
-  const node = document.getElementById(NODE_ID);
+  const node = document.getElementById( remark_config.node || NODE_ID);
 
   if (!node) {
     console.error("Remark42: Can't find root node.");