Persiting items using shared_preferences

linked to #18

At the moment we can create new task and toggle then from uncomplete to complete.
However the state of the application is not saved and each time the application is restarted the items created are lost.

Using the shared preferences package we can save a serialized version of the application state (using json).
- [x] The first step is to know how to initialise the state of statefull widgets with [initState](https://api.flutter.dev/flutter/widgets/State/initState.html)
- [x] Once the list of items is loaded from the disk we will need to save the state each time an item is created and each time an item is toggled.

see: https://pub.dev/packages/shared_preferences