Add autoplay URL parameter trigger. (#764)

Resolves #720

src/plugins/README.md

@@ -14,7 +14,8 @@ rather require the user to invoke them somehow. For example:
 * The *navigation* plugin waits for the user to press some keys, arrows, page
   down, page up, space or tab.
 * The *autoplay* plugin looks for the HTML attribute `data-autoplay` to see
-  whether it should do its thing.
+  whether it should do its thing. It can also be triggered with a URL GET parameter
+  `?impress-autoplay=5` *5 is the waiting duration*. 
 * The *toolbar* plugin looks for a `<div>` element to become visible.
 
 Extra addons

src/plugins/autoplay/autoplay.js

@@ -31,9 +31,10 @@
         // Element attributes starting with "data-", become available under
         // element.dataset. In addition hyphenized words become camelCased.
         var data = root.dataset;
+        var autoplay = util.getUrlParamValue( "impress-autoplay" ) || data.autoplay;
 
-        if ( data.autoplay ) {
-            autoplayDefault = util.toNumber( data.autoplay, 0 );
+        if ( autoplay ) {
+            autoplayDefault = util.toNumber( autoplay, 0 );
         }
 
         var toolbar = document.querySelector( "#impress-toolbar" );