The Todo list expanded

1. A tasks array property
2. A method printTasks that will, like the previous excercise, loop over the TodoList's tasks and print similarly.
3. An addTask method that will add a given task to the list of tasks.
4. Use the object to add a task to the TodoList and then print the tasks
5. Bonus: add a completeTask method that will take an task_index as a parameter and will set the appropriate task to be done