Add progress bar plugin by Matthias Bilger (@m42e).

Originally from https://github.com/m42e/impress.js-progress and
adapted for the new plugin api. Also made the sample CSS produce
a smaller bar and font.

Adds event.detail.next to impress:stepleave event in impress.js.

src/plugins/progress/progress.js

@@ -0,0 +1,58 @@
+/* global document */
+( function( document ) {
+    "use strict";
+    var root;
+    var stepids = [];
+
+    // Get stepids from the steps under impress root
+    var getSteps = function() {
+        stepids = [];
+        var steps = root.querySelectorAll( ".step" );
+        for ( var i = 0; i < steps.length; i++ )
+        {
+          stepids[ i + 1 ] = steps[ i ].id;
+        }
+        };
+
+    // Wait for impress.js to be initialized
+    document.addEventListener( "impress:init", function( event ) {
+            root = event.target;
+        getSteps();
+        var gc = event.detail.api.lib.gc;
+        gc.addCallback( function() {
+            stepids = [];
+            if ( progressbar ) {
+                progressbar.style.width = "";
+                        }
+            if ( progress ) {
+                progress.innerHTML = "";
+                        }
+        } );
+    } );
+
+    var progressbar = document.querySelector( "div.impress-progressbar div" );
+    var progress = document.querySelector( "div.impress-progress" );
+
+    if ( null !== progressbar || null !== progress ) {
+        document.addEventListener( "impress:stepleave", function( event ) {
+            updateProgressbar( event.detail.next.id );
+        } );
+
+        document.addEventListener( "impress:steprefresh", function( event ) {
+            getSteps();
+            updateProgressbar( event.target.id );
+        } );
+
+    }
+
+    function updateProgressbar( slideId ) {
+        var slideNumber = stepids.indexOf( slideId );
+        if ( null !== progressbar ) {
+                        var width = 100 / ( stepids.length - 1 ) * ( slideNumber );
+            progressbar.style.width = width.toFixed( 2 ) + "%";
+        }
+        if ( null !== progress ) {
+            progress.innerHTML = slideNumber + "/" + ( stepids.length - 1 );
+        }
+    }
+} )( document );