Support load of extensions without specifying trailing $, see #2496

* Backwards compatible, i.e. still possible to use $
* DynamicAccess tries first with $ in getObjectFor, then falls back
  to fqcn as is

akka-docs/scala/code/docs/extension/ExtensionDocSpec.scala

@@ -45,7 +45,7 @@ object ExtensionDocSpec {
   val config = """
     //#config
     akka {
-      extensions = ["docs.extension.CountExtension$"]
+      extensions = ["docs.extension.CountExtension"]
     }
     //#config
     """