Preventing the user shooting themself in the foot

[Robin Green]

It's really dangerous to use the 'a' key in notmuch-mode in an inbox
thread which has multiple unread replies! Yes, the other unread replies
will still be tagged unread, but the user might not immediately be aware
of them. It would be really useful to have an optional warning ("More
unread messages in this thread, are you sure?") for this situation!