Add new flutter example of todo app with provider (#78)

todo_list_using_provider/lib/models/todo.dart

@@ -0,0 +1,16 @@
+import 'package:flutter/material.dart';
+
+class Todo extends ChangeNotifier {
+  final String id;
+  String title;
+  bool done;
+
+  Todo({@required this.id, this.title, this.done = false});
+
+
+  /// Toggles the value of the item and notify to listeners
+  void toggle() {
+    this.done = !this.done;
+    notifyListeners();
+  }
+}