Add the configuration option "jump.grepCmd" that allows to configure the
command that is used to search in grep mode. This allows the users of
git-jump to use ag(1) or ack(1) as search engines.
Signed-off-by: Beat Bolli <dev+git@drbeat.li> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>