Use 'todo' controller for task list items
Load 'todo' controller script

The current iterating block is using the model directly, this will bind
it to the controller instead.
Add 'todo' controller

The controller returns 'isCompleted' as a property; its method manipul-
ates the model. It first assigns the model to the 'model' variable with
a getter, then checks to see if the method was passed a 'value' argument.

If it wasn't, it will return the model's current value of 'isCompleted'.

If it is, the model will update 'isCompleted' with the value of 'value'.
Afterwards, the model is explicitly saved, and the 'value' returned.