- Add mutable configuration provider

- Split tests file
- Fixed and nullable functions
- Able to set or remove config from system config

example/example.dart

@@ -1,3 +1,4 @@
+import 'package:configurable/configurable.dart';
 import 'package:configurable/dotenv_configuration_provider.dart';
 import 'package:configurable/simple_configuration_provider.dart';
 import 'package:configurable/system_config.dart';
@@ -6,18 +7,27 @@ void main() {
   var key = 'app.name';
   var value = 'CUBETIQ Solution';
 
-  // in-memory provider (built-in)
+  /// in-memory provider (built-in)
   var simpleProvider = SimpleConfigurationProvider();
   SystemConfig.setProvider(simpleProvider);
   var result1 = SystemConfig.getOrNull(key, defaultValue: value);
-  // output: CUBETIQ Solution
+
+  /// output: CUBETIQ Solution
   print(result1);
 
-  // dotenv provider (from file .env)
+  /// get config functions
+  getConfig('app.title', defaultValue: 'My App Title!');
+  getConfigOrNull('app.null.ignore');
+
+  /// set config functions
+  /// support only mutable configuration provider
+  setConfig('my.app', 'Hello My App');
+
+  /// dotenv provider (from file .env)
   var dotenvProvider = DotenvConfigurationProvider();
   SystemConfig.setProvider(dotenvProvider);
   var result2 = SystemConfig.getOrNull('HOME');
 
-  // output: user's home directory
+  /// output: user's home directory
   print(result2);
-}
+}