Allow the -f option to force clean orphan snapshots

[jessepiccolo]

If the user uses the -f option, this should forcefully clean the snapshots as well.

[GMartinez-Sisti]

I would like this option to work also.

For now I'm using echo "y" | amicleaner --check-orphans in automation.

[cloudlena]

I think #66 would solve this, right? Can we get it merged, @bonclay7?