Upgrade from Shimmer to Squirrel.
Update NuGet packages.
Add ReactiveUI and basic view models.
Use RxUI WhenAny.
Add initial notification support.

First notification is a prompt to open a file (if the app is launched with no/invalid command-line arguments).
Move view models to new namespace.
Invert age colouring.

Now: darker is older, and lighter is newer.
Expose the current top line number in the view.
Add automatic updating.
Ignore TimeoutException when checking for updates.