Add saving to and reading from a file

To make sure when our timer runs it runs

pubspec.yaml

@@ -34,6 +34,7 @@ dependencies:
   # The following adds the Cupertino Icons font to your application.
   # Use with the CupertinoIcons class for iOS style icons.
   cupertino_icons: ^1.0.8
+  path_provider: ^2.1.2  # For accessing local file system
 
 dev_dependencies:
   flutter_test: