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/README.md

@@ -0,0 +1,46 @@
+Progress plugin
+===============
+
+Progressbar and pagexounter for impress.js presentations
+
+Usage
+-----
+
+Add a div for progressbar and/or progress as you can see it here:
+
+### HTML
+
+	  <div class="impress-progressbar"><div></div></div>
+	  <div class="impress-progress"></div>
+
+### Sample CSS
+
+    .impress-progressbar {
+      position: absolute;
+      right: 318px;
+      bottom: 1px;
+      left: 118px;
+      border-radius: 7px;
+      border: 2px solid rgba(100, 100, 100, 0.2);
+    }
+    .impress-progressbar DIV {
+      width: 0;
+      height: 2px;
+      border-radius: 5px;
+      background: rgba(75, 75, 75, 0.4);
+      transition: width 1s linear;
+    }
+    .impress-progress {
+      position: absolute;
+      left: 59px;
+      bottom: 1px;
+      text-align: left;
+      opacity: 0.6;
+    }
+
+Feel free to change the style of your progressbar as you like by editing the CSS file.
+
+Author
+------
+
+Copyright 2014: Matthias Bilger (@m42e)