Adding more details to PromptDialog

- replacing the default EventArgs within the Completed event with a more detailed one
- adding new PromptDialogCompletedEventArgs class for easier details access

TelegramBotBase/Args/PromptDialogCompletedEventArgs.cs

@@ -0,0 +1,14 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace TelegramBotBase.Args
+{
+    public class PromptDialogCompletedEventArgs
+    {
+        public object Tag { get; set; }
+
+        public String Value { get; set; }
+
+    }
+}