Do not register REPL commands if REPL is not being used.

This prevents an error in MonoDevelop when all assemblies in the
current AppDomain have their exported types checked for any
types that implement IReplCommand.

MonoDevelop error reported:

System.IO.FileNotFoundException: Could not load file or assembly
'MonoDevelop.TextTemplating, Version=2.6.0.0, Culture=neutral,
PublicKeyToken=null' or one of its dependencies. The system cannot
find the file specified.
File name: 'MonoDevelop.TextTemplating, Version=2.6.0.0,
Culture=neutral, PublicKeyToken=null'
   at System.Reflection.RuntimeAssembly.GetExportedTypes(
RuntimeAssembly assembly, ObjectHandleOnStack retTypes)
   at System.Reflection.RuntimeAssembly.GetExportedTypes()
   at ScriptCs.Hosting.RuntimeServices.<RegisterReplCommands>b__27(Assembly x)
   at System.Linq.Enumerable.<SelectManyIterator>d__14`2.MoveNext()
   at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
   at System.Linq.Buffer`1..ctor(IEnumerable`1 source)
   at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
   at ScriptCs.Hosting.RuntimeServices.RegisterReplCommands(
ContainerBuilder builder)
   at ScriptCs.Hosting.RuntimeServices.CreateContainer()
   at ScriptCs.Hosting.ScriptServicesRegistration.get_Container()
   at ScriptCs.Hosting.RuntimeServices.GetScriptServices()
   at ScriptCs.Hosting.ScriptServicesBuilder.Build()
   at MonoDevelop.PackageManagement.ScriptCsHost.RunScript(String fileName)