Add option to make notification persist, like (-u critical)

[poetaman]

Thanks for creating this! As you already know, arttime project relies on wsl-notify-send for notifications on Windows. The one feature that would be good to have is notifications that don't timeout. notify-send has an option -u critical for that. Having this one missing feature would make wsl-notify-send more adoptable, as being able to control the timeout behavior of notifications is quite important.