Add Mobile plugin, which allows authors to hide most slides with CSS.

The Mobile plugin adds CSS classes body.impress-mobile and
div.prev, div.next. These can be used in CSS to hide non-active
steps completely, in order to reduce memory consumption on
small mobile devices.

src/plugins/mobile/README.md

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+Mobile devices support
+======================
+
+Presentations with a lot of 3D effects and graphics can consume a lot of resources, especially on mobile devices.
+This plugin provides some CSS classes that can be used to hide most of the slides, only showing the current, previous
+and next slide.
+
+In particular, this plugin adds:
+
+`body.impress-mobile` class, if it detects running on a mobile OS.
+
+`div.prev` and `div.prev` to the adjacent steps to the current one. Note that the current slide is already identified
+by `present` and `active` CSS classes.
+
+Example CSS
+-----------
+
+        body.impress-mobile .step { 
+            display:none;
+        }
+        body.impress-mobile .step.active,
+        body.impress-mobile .step.present,
+        body.impress-mobile .step.next,
+        body.impress-mobile .step.prev { 
+            display:block; 
+        }
+
+Note
+----
+
+This plugin does not take into account redirects that could happen with skip, goto and other plugins. The active
+step will of course always be correct, but "non-linear" transitions to anything else than the actual previous and next
+steps will probably not look correct.
+
+Author
+------
+
+Kurt Zenisek (@KZeni)

src/plugins/mobile/mobile.js

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+/**
+ * Mobile devices support
+ *
+ * Allow presentation creators to hide all but 3 slides, to save resources, particularly on mobile
+ * devices, using classes body.impress-mobile, .step.prev, .step.active and .step.next.
+ *
+ * Note: This plugin does not take into account possible redirections done with skip, goto etc
+ * plugins. Basically it wouldn't work as intended in such cases, but the active step will at least
+ * be correct.
+ *
+ * Adapted to a plugin from a submission by @Kzeni:
+ * https://github.com/impress/impress.js/issues/333
+ */
+/* global document, navigator */
+( function( document ) {
+    "use strict";
+
+    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 ];
+                }
+            }
+        }
+    };
+
+    // Detect mobile browsers & add CSS class as appropriate.
+    document.addEventListener( "impress:init", function( event ) {
+        var body = document.body;
+        if ( /Android|webOS|iPhone|iPad|iPod|BlackBerry|IEMobile|Opera Mini/i.test(
+                 navigator.userAgent
+             ) ) {
+            body.classList.add( "impress-mobile" );
+        }
+
+        // Unset all this on teardown
+        var api = event.detail.api;
+        api.lib.gc.addCallback( function() {
+            document.body.classList.remove( "impress-mobile" );
+            var prev = document.getElementsByClassName( "prev" )[ 0 ];
+            var next = document.getElementsByClassName( "next" )[ 0 ];
+            if ( typeof prev !== "undefined" ) {
+                prev.classList.remove( "prev" );
+            }
+            if ( typeof next !== "undefined" ) {
+                next.classList.remove( "next" );
+            }
+        } );
+    } );
+
+    // Add prev and next classes to the siblings of the newly entered active step element
+    // Remove prev and next classes from their current step elements
+    // Note: As an exception we break namespacing rules, as these are useful general purpose
+    // classes. (Naming rules would require us to use css classes mobile-next and mobile-prev,
+    // based on plugin name.)
+    document.addEventListener( "impress:stepenter", function( event ) {
+	      var oldprev = document.getElementsByClassName( "prev" )[ 0 ];
+	      var oldnext = document.getElementsByClassName( "next" )[ 0 ];
+
+	      var prev = getPrevStep( event.target );
+	      prev.classList.add( "prev" );
+	      var next = getNextStep( event.target );
+	      next.classList.add( "next" );
+
+	      if ( typeof oldprev !== "undefined" ) {
+		      oldprev.classList.remove( "prev" );
+              }
+	      if ( typeof oldnext !== "undefined" ) {
+		      oldnext.classList.remove( "next" );
+              }
+    } );
+} )( document );
+