Add skip plugin

Add class="skip" to a step to cause the step to be skipped when nagivating
with prev() and next(). A goto() to the step will still work.

src/plugins/skip/README.md

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+Skip Plugin
+===========
+
+Example:
+
+        <!-- This slide is disabled in presentations, when moving with next()
+             and prev() commands, but you can still move directly to it, for
+             example with a url (anything using goto()). -->
+        <div class="step skip">
+
+The skip plugin is a pre-stepleave plugin. It is executed before 
+`impress:stepleave` event. If the next step also has `class="skip"`
+set, it will set the next step to the one after that.
+
+Author
+------
+
+Copyright 2016 Henrik Ingo (@henrikingo)
+Released under the MIT license.
+

src/plugins/skip/skip.js

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+/**
+ * Skip Plugin
+ *
+ * Example:
+ *
+ *    <!-- This slide is disabled in presentations, when moving with next()
+ *         and prev() commands, but you can still move directly to it, for
+ *         example with a url (anything using goto()). -->
+ *         <div class="step skip">
+ *
+ * Copyright 2016 Henrik Ingo (@henrikingo)
+ * Released under the MIT license.
+ */
+
+/* global document, window */
+
+( function( document, window ) {
+    "use strict";
+    var util;
+
+    document.addEventListener( "impress:init", function( event ) {
+        util = event.detail.api.lib.util;
+    }, false );
+
+    var getNextStep = function( el ) {
+        var steps = document.querySelectorAll( ".step" );
+        for ( var i = 0; i < steps.length; i++ ) {
+            if ( steps[ i ] === el ) {
+                if ( i + 1 < steps.length ) {
+                    return steps[ i + 1 ];
+                } else {
+                    return steps[ 0 ];
+                }
+            }
+        }
+    };
+    var getPrevStep = function( el ) {
+        var steps = document.querySelectorAll( ".step" );
+        for ( var i = steps.length - 1; i >= 0; i-- ) {
+            if ( steps[ i ] === el ) {
+                if ( i - 1 >= 0 ) {
+                    return steps[ i - 1 ];
+                } else {
+                    return steps[ steps.length - 1 ];
+                }
+            }
+        }
+    };
+
+    var skip = function( event ) {
+        if ( ( !event ) || ( !event.target ) ) {
+            return;
+        }
+
+        if ( event.detail.next.classList.contains( "skip" ) ) {
+            if ( event.detail.reason === "next" ) {
+
+                // Go to the next next step instead
+                event.detail.next = getNextStep( event.detail.next );
+
+                // Recursively call this plugin again, until there's a step not to skip
+                skip( event );
+            } else if ( event.detail.reason === "prev" ) {
+
+                // Go to the previous previous step instead
+                event.detail.next = getPrevStep( event.detail.next );
+                skip( event );
+            }
+
+            // If the new next element has its own transitionDuration, we're responsible for setting
+            // that on the event as well
+            event.detail.transitionDuration = util.toNumber(
+                event.detail.next.dataset.transitionDuration, event.detail.transitionDuration
+            );
+        }
+    };
+
+    // Register the plugin to be called in pre-stepleave phase
+    // The weight makes this plugin run early. This is a good thing, because this plugin calls
+    // itself recursively.
+    window.impress.addPreStepLeavePlugin( skip, 1 );
+
+} )( document, window );
+