Add a cross-platform tool for desktop notifications using --post-run-command

Building on #74 (#76) it would be nice to have a cross-platform way of using desktop notifications.

I'd like to avoid additional golang dependencies in `gotestsum` and use the `os/exec` approach for running the notification.

A couple options that come to mind:
1. If there are widely used CLI tools (such as `terminal-notifier` in `contrib/notify/notify-macos.go`) we could add a new `cmd/tool/notify` which detects which CLI is available and shells out to the correct CLI
2. We could build a separate binary using something like https://github.com/martinlindhe/notify and make it available to download from the github releases page.