Add configurable with functions

Add more tests for call functions and fixed

test/config_test.dart

@@ -1,3 +1,5 @@
+import 'package:configurable/configurable.dart' show getConfigOrNull;
+import 'package:configurable/dotenv_configuration_provider.dart';
 import 'package:configurable/system_config.dart';
 import 'package:test/test.dart';
 
@@ -10,5 +12,27 @@ void main() {
     expect(value, equals(result));
     expect(value, equals(SystemConfig.getOrNull(key)));
   });
-  
+
+  test('get system config by key with dotenv provider', () {
+    var key = 'app.name';
+    var value = 'CUBETIQ Solution';
+
+    // set dotenv provider
+    SystemConfig.setProvider(DotenvConfigurationProvider());
+
+    var result = SystemConfig.getOrNull(key);
+
+    expect(value, equals(result));
+    expect(value, equals(SystemConfig.getOrNull(key)));
+  });
+
+  test('get config by key with function', () {
+    var key = 'app.name';
+    var value = 'CUBETIQ Solution';
+
+    var result = getConfigOrNull(key, defaultValue: value);
+
+    expect(value, equals(result));
+    expect(value, equals(getConfigOrNull(key)));
+  });
 }