Add SaveToFile in the mapping (#284)

* savetofile

* fix !

src/WireMock.Net/IMapping.cs

@@ -1,6 +1,7 @@
-using System;
+using JetBrains.Annotations;
+using System;
 using System.Threading.Tasks;
-using JetBrains.Annotations;
+using WireMock.Handlers;
 using WireMock.Matchers.Request;
 using WireMock.ResponseProviders;
 
@@ -60,6 +61,11 @@ namespace WireMock
         /// </summary>
         IResponseProvider Provider { get; }
 
+        /// <summary>
+        /// The FileSystemHandler.
+        /// </summary>
+        IFileSystemHandler FileSystemHandler { get; }
+
         /// <summary>
         /// Is State started ?
         /// </summary>