Add support for visual notifications on Windows to the `notify` task

I wrote a Windows version of a command-line desktop notification program (https://github.com/RadicalZephyr/DesktopNotifier), so the `notify` task could try to check for the presence of this program the way it does for the Mac and Linux programs.

This isn't a PR yet because I need to figure out what it looks like to check for the existence of that program on Windows (any Windows developers feel free to chime in).