add option to run a script prior to exiting umad

[erikng]

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.

[poundbangbash]

Attempted resolution in https://github.com/erikng/umad/pull/21