Pull notification logic; use generalized ding package.

[c4lliope]

Hello! I'd been looking around to see how I could easily pair notify-send and https://hexdocs.pm/desktop

Your code inside ExUnitNotifier.Notifiers seemed like a really good basis for a cross-OS notifications pkg.
I expended a couple hours to pull the ExUnit logic back into a single module,
and increased the number of options you can pass along to the handlers / notifiers.

I hope this is a clean approach and can help more people use the logic you prepared.
I'm also eager and happy to share push permissions on the ding library on hex.pm,
since you inspired the code.

This PR is optional in case you'd like to cleanly depend on ding as a dependency.
This codebase can become more minimal, keeping a focus on ExUnit and how the ding options are prepared.
The branch needs to be checked using terminal-notify before merging though,
I checked as much as I could on Linux, and tmux-notifier / notify-send/ terminal-title all look good.