Added ExportFormat Class

Author: DavidSuescunPelegay

src/jQueryDatatableServerSideNetCore/Data/ExportFormat.cs

@@ -0,0 +1,12 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+
+namespace jQueryDatatableServerSideNetCore.Data
+{
+    public class ExportFormat
+    {
+        public const string Excel = "excel";
+    }
+}