Option to purge trash of all users when running as root

[Jackenmen]

After making issue #47, I realised that autotrash doesn't actually take out purge other user's trash when using --trash-mounts option. If I knew that I would have probably waited with asking about the snap package until this feature is implemented (and if it's accepted, to begin with of course) since it kinda doesn't make as much sense to create a snap package if all of the functionality is local to the user that runs autotrash command rather than all users.

Anyway, I think it would be great to have an option to purge trash of all users so that one could run autotrash as a system service and have it take care of everyone's Trash automatically. It's way easier than setting up a separate service for each user and also makes autotrash actually run for users that you're not logged into as opposed to user services that only run while you're in a user session.