Added stickyClass (css class) feature.

Author: vovayatsyuk

lib/web/mage/sticky.js

@@ -12,7 +12,8 @@ define([
     $.widget('mage.sticky', {
         options: {
             container: '',
-            spacingTop: 0
+            spacingTop: 0,
+            stickyClass: '_sticky'
         },
 
         /**
@@ -51,7 +52,9 @@ define([
 
                 offset = Math.max( 0, Math.min( offset, this.maxOffset) );
 
-                this.element.css( 'top', offset );
+                this.element
+                    .toggleClass(this.options.stickyClass, (offset > 0))
+                    .css( 'top', offset );
             }
         },