send-email: also pick up cc addresses from -by trailers

send-email: also pick up cc addresses from -by trailers

When rerolling a patch series, including various Reviewed-by etc. that
may have come in, it is quite convenient to have git-send-email
automatically cc those people.

So pick up any *-by lines, with a new suppression category 'misc-by',
but special-case Signed-off-by, since that already has its own
suppression category. It seems natural to make 'misc-by' implied by
'body'.

Based-on-patch-by: Joe Perches <joe@perches.com>
Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>