Readers may lose interest in subscription topics. This lets
them avoid clutter by forgetting a saved search.
This does not and will not destroy the contents of an --output
mailbox. In other words, this is similar to unsubscribing
from an Atom/RSS feed or NNTP group.
I've also decided we won't support 'mv-search', since it'll
probably be rarely used and "lei convert" can be used, instead.