lei q: add --mua-cmd switch

lei q: add --mua-cmd switch

It can be convenient to invoke an MUA as search results
are being written to it, as an eager person may want to
start seeing results ASAP.  This lets Maildir users
see results in the MUA as we are writing them.  Users
of IMAP will eventually be able to take advantage of
them, too.

Since we don't support mbox locking (yet?), we'll only invoke
the MUA after results are done for mbox formats.