Handlebars Extension (#286)

* wip

* HandlebarsRegistrationCallback

* HandlebarsContextFactoryTests

* 1.0.21.0

* fix sonar

* LocalFileSystemHandler

* readme

* Fix System.IO.IOException

src/WireMock.Net/Settings/IFluentMockServerSettings.cs

@@ -1,5 +1,6 @@
-using System;
+using HandlebarsDotNet;
 using JetBrains.Annotations;
+using System;
 using WireMock.Handlers;
 using WireMock.Logging;
 
@@ -112,5 +113,11 @@ namespace WireMock.Settings
         /// </summary>
         [PublicAPI]
         IFileSystemHandler FileSystemHandler { get; set; }
+
+        /// <summary>
+        /// Action which can be used to add additional is Handlebar registrations. [Optional]
+        /// </summary>
+        [PublicAPI]
+        Action<IHandlebars, IFileSystemHandler> HandlebarsRegistrationCallback { get; set; }
     }
 }