Pass message to the `notmuch-show-insert-text/plain-hook' hook.

Pass message to the `notmuch-show-insert-text/plain-hook' hook.

Before the change, the `notmuch-show-insert-text/plain-hook' was
given only the `depth' argument.  The patch adds another one -
the message.  Currently, the new message argument is not used by
any on the hooks.  But it will be used later to get access to
message invisibility specs when wash buttons are inserted.