convert plugins to ES modules, transpile es5 versions backwards compatibility

js/controllers/plugins.js

@@ -60,8 +60,8 @@ export default class Plugins {
 
 				// Load synchronous scripts
 				scripts.forEach( s => {
-					if( s.plugin ) {
-						this.registerPlugin( s.plugin );
+					if( typeof s.id === 'string' ) {
+						this.registerPlugin( s );
 						scriptLoadedCallback( s );
 					}
 					else {

js/reveal.js

@@ -28,7 +28,6 @@ import {
 	deserialize,
 	transformElement,
 	createSingletonNode,
-	closestParent,
 	getQueryHash,
 	getRemainingHeight
 } from './utils/util.js'