Add example and add more tests and fixed function

example/example.dart

@@ -1,5 +1,6 @@
 import 'package:configurable/dotenv_configuration_provider.dart';
 import 'package:configurable/simple_configuration_provider.dart';
+import 'package:configurable/system_config.dart';
 
 void main() {
   var key = 'app.name';
@@ -7,9 +8,16 @@ void main() {
 
   // in-memory provider (built-in)
   var simpleProvider = SimpleConfigurationProvider();
+  SystemConfig.setProvider(simpleProvider);
+  var result1 = SystemConfig.getOrNull(key, defaultValue: value);
+  // output: CUBETIQ Solution
+  print(result1);
 
   // dotenv provider (from file .env)
   var dotenvProvider = DotenvConfigurationProvider();
+  SystemConfig.setProvider(dotenvProvider);
+  var result2 = SystemConfig.getOrNull('HOME');
 
-  
+  // output: user's home directory
+  print(result2);
 }