add option to run a script prior to exiting umad

if someone has an API script or some other kind of script they want to run before UMAD exits, we should open this up.

It would have to be ran only as the currently logged in user though to not create a root privilege escalation.

No files, tests, or entry points are named. Start by locating UMAD's exit path and documenting how a pre-exit script would be configured and invoked; done means the script runs before exit as the currently logged-in user without root privilege escalation.

Written by the indexing model from the issue text.