Move signal logging to the PostHook.

This change lets people use a custom logger without getting extraneous output on signals.

It also serves as a demo in how to use the hook functions. :)
I think this would make more sense as a `PreHook`: if we hang during the shutdown process (e.g., we're busy serving a slow request or something) we still want the message to be printed so the user knows their C-c was acknowledged.