notmuch: Make the command of "notmuch" walk the user through the next step.

If this is run first, it will run "notmuch setup" directly. After that
is successful, it will look for a databae and tell the user to run
"notmuch new" if the database doesn't exist yet. Finally, if the
database is present, it will provide some example "notmuch search"
commands for the user to try.