I'd like to be able to set .card margin-bottom indepedently of .card-header padding
Created by: jcoyne
Currently .card
margin-bottom
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_card.scss#L8
and .card-header
padding
https://github.com/twbs/bootstrap/blob/v4-dev/scss/_card.scss#L76
both use the $card-spacer-y
variable.