Added onChanging callback to accordion

Because it's useful

dist/semantic.js

@@ -2374,6 +2374,7 @@ $.fn.accordion = function(parameters) {
           }
           module.debug('Opening accordion content', $activeTitle);
           settings.onOpening.call($activeContent);
+          settings.onChanging.call($activeContent);
           if(settings.exclusive) {
             module.closeOthers.call($activeTitle);
           }
@@ -2437,6 +2438,7 @@ $.fn.accordion = function(parameters) {
           if((isActive || isOpening) && !isClosing) {
             module.debug('Closing accordion content', $activeContent);
             settings.onClosing.call($activeContent);
+            settings.onChanging.call($activeContent);
             $activeTitle
               .removeClass(className.active)
             ;
@@ -2784,6 +2786,7 @@ $.fn.accordion.settings = {
   onOpen          : function(){}, // callback after open animation
   onClosing       : function(){}, // callback before closing animation
   onClose         : function(){}, // callback after closing animation
+  onChanging      : function(){}, // callback before closing or opening animation
   onChange        : function(){}, // callback after closing or opening animation
 
   error: {