Implemented async saving and loading

Filtration.Interface/IDocument.cs

@@ -1,9 +1,11 @@
-namespace Filtration.Interface
+using System.Threading.Tasks;
+
+namespace Filtration.Interface
 {
     public interface IDocument
     {
         bool IsScript { get; }
         bool IsTheme { get; }
-        void Close();
+        Task Close();
     }
 }