[feature-request] allow to add custom script or commands to base builder

[TLINDEN]

That way the admin could easily script any more tasks and execute them in a newly build or updated jail without the need to manually copy/mount/install it each time.

Also it must be possible to add such stuff in hindsight, that is after a jail is already running, possibly for all jails or all jails matching a pattern.

You get the idea.