support including plugins via <script> instead of dependencies

js/index.js

@@ -45,7 +45,7 @@ window.Reveal.initialize = options => {
  * queuing up early API calls and invoking all of them
  * when Reveal.initialize is called.
  */
-[ 'on', 'off', 'addEventListener', 'removeEventListener' ].forEach( method => {
+[ 'on', 'off', 'addEventListener', 'removeEventListener', 'registerPlugin' ].forEach( method => {
 	window.Reveal[method] = ( ...args ) => {
 		enqueuedAPICalls.push( deck => deck[method].call( null, ...args ) );
 	}

js/reveal.js

@@ -34,7 +34,7 @@ import {
 
 /**
  * reveal.js
- * http://revealjs.com
+ * https://revealjs.com
  * MIT licensed
  *
  * Copyright (C) 2020 Hakim El Hattab, https://hakim.se