Handlebars Extension (#286)

* wip

* HandlebarsRegistrationCallback

* HandlebarsContextFactoryTests

* 1.0.21.0

* fix sonar

* LocalFileSystemHandler

* readme

* Fix System.IO.IOException

src/WireMock.Net/Util/FileHelper.cs

@@ -1,6 +1,5 @@
-using System.IO;
+using JetBrains.Annotations;
 using System.Threading;
-using JetBrains.Annotations;
 using WireMock.Handlers;
 using WireMock.Validation;
 
@@ -11,16 +10,19 @@ namespace WireMock.Util
         private const int NumberOfRetries = 3;
         private const int DelayOnRetry = 500;
 
-        public static string ReadAllTextWithRetryAndDelay([NotNull] IFileSystemHandler handler, [NotNull] string path)
+        public static bool TryReadMappingFileWithRetryAndDelay([NotNull] IFileSystemHandler handler, [NotNull] string path, out string value)
         {
             Check.NotNull(handler, nameof(handler));
             Check.NotNullOrEmpty(path, nameof(path));
 
+            value = null;
+
             for (int i = 1; i <= NumberOfRetries; ++i)
             {
                 try
                 {
-                    return handler.ReadMappingFile(path);
+                    value = handler.ReadMappingFile(path);
+                    return true;
                 }
                 catch
                 {
@@ -28,7 +30,7 @@ namespace WireMock.Util
                 }
             }
 
-            throw new IOException();
+            return false;
         }
     }
 }