cli: add support for replying just to the sender in "notmuch reply"

Add new option --reply-to=(all|sender) to "notmuch reply" to select whether
to reply to all (sender and all recipients), or just sender. Reply to all
remains the default.

Credits to Mark Walters <markwalters1009@gmail.com> for his similar earlier
work where I picked up the basic idea of handling reply-to-sender in
add_recipients_from_message(). All bugs are mine, though.

Signed-off-by: Jani Nikula <jani@nikula.org>