Clear notification once you've tapped the yubikey

[psanford]

On linux, you can get the notification id when you trigger a notification so you can clear it later instead of just depending on a timeout. Presumably there's a way to also do this on macOS.

I've got a patch that implements this on linux, if there is interest I can turn this into a PR: https://github.com/psanford/yubikey-agent/commit/9ec54aeb20ce93bfbccd036b435225db9a48713f

[catdevnull]

I didn't see this when implementing #141 :)