Add configurable with functions

Add more tests for call functions and fixed

example/example.dart

@@ -1,3 +1,15 @@
+import 'package:configurable/dotenv_configuration_provider.dart';
+import 'package:configurable/simple_configuration_provider.dart';
+
 void main() {
-  print('Example');
+  var key = 'app.name';
+  var value = 'CUBETIQ Solution';
+
+  // in-memory provider (built-in)
+  var simpleProvider = SimpleConfigurationProvider();
+
+  // dotenv provider (from file .env)
+  var dotenvProvider = DotenvConfigurationProvider();
+
+  
 }