Add a framework for synchronously executed preInit and preStepLeave plugins.

This allows plugins to register to be executed at the beginning of
impress().init() and impress().goto() respectively. By returning false,
a plugin can also cancel the event.

Also adds 3 plugins that use this: rel, goto and stop.

src/plugins/stop/README.md

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+Stop Plugin
+===========
+
+Example:
+
+        <!-- Stop at this slide.
+             (For example, when used on the last slide, this prevents the 
+             presentation from wrapping back to the beginning.) -->
+        <div class="step stop">
+
+The stop plugin is a pre-stepleave plugin. It is executed before 
+`impress:stepleave` event. If the current slide has `class="stop"`
+set, it will disable the next() command by setting the next slide to the current
+slide.
+
+Author
+------
+
+Copyright 2016 Henrik Ingo (@henrikingo)
+Released under the MIT license.
+

src/plugins/stop/stop.js

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+/**
+ * Stop Plugin
+ *
+ * Example:
+ *
+ *        <!-- Stop at this slide.
+ *             (For example, when used on the last slide, this prevents the
+ *             presentation from wrapping back to the beginning.) -->
+ *        <div class="step stop">
+ *
+ * Copyright 2016 Henrik Ingo (@henrikingo)
+ * Released under the MIT license.
+ */
+/* global document, window */
+( function( document, window ) {
+    "use strict";
+
+    var stop = function( event ) {
+        if ( ( !event ) || ( !event.target ) ) {
+            return;
+        }
+
+        if ( event.target.classList.contains( "stop" ) ) {
+            if ( event.detail.reason === "next" ) {
+                return false;
+            }
+        }
+    };
+
+    // Register the plugin to be called in pre-stepleave phase
+    // The weight makes this plugin run fairly early.
+    window.impress.addPreStepLeavePlugin( stop, 2 );
+
+} )( document, window );
+