Add 'before send request' js callbacks (#137)

Add 'before send request' js callbacks

Author: luizbills

assets/scripts/wplf-form.js

@@ -5,6 +5,7 @@
 window.wplf = {
   successCallbacks: [],
   errorCallbacks: [],
+  beforeSendCallbacks: [],
   submitHandler: function (e) {
     var form = e.target;
     var data = new FormData(form);
@@ -25,6 +26,10 @@ window.wplf = {
       // reset errors
       error.parentNode.removeChild(error);
     });
+    
+    window.wplf.beforeSendCallbacks.forEach(function(func) {
+      func(form);
+    });
 
     fetch(ajax_object.ajax_url, {
       method: "POST",