Added feature "Increase Due Date By 1 Day" (`Ctrl+Alt+Up`) and "Decrease Due Date By 1 Day" (`Ctrl+Alt+Down`) commands. Closes Issue #203.

Removed "PreviewKeyDown" event handler from MainWindowViewModel.cs. Moved logic to RoutedUICommands "EmulateUpArrow" and "EmulateDownArrow" defined in MainWindow.xaml and implemented in MainWindowViewModel.cs. Part of refactor discussed Issue #200. This change helps keep all keyboard shortcuts defined the same way.

Updated list of keyboard shortcuts in README.md and in help text.