Create autoremove() task

Sometimes, the kernel maintenance tasks find packages that are no longer
required. When this occurs, they just spit this out to stdout, but don't
clean them up.

It would be a good idea to clean them up, so allow this to happen using a
new autoremove() task. Run said autoremove() task as part of the
remove_unused() task, but additionally allow it to be run manually.

If there are no packages to be removed using apt-get autoremove, this should
be a no-op, else it will remove packages specified.