Use mixin to generate card color variations

Created by: bassjobsen

i found that in the current version the cards module has not got card-variant() mixins, just like the button module. Compare: https://github.com/twbs/bootstrap/blob/v4-dev/scss/_card.scss#L90 and https://github.com/twbs/bootstrap/blob/v4-dev/scss/_buttons.scss#L56 Of course i understand the card variant only set two properties, but on the other hand not having these mixins seems inconsequent and not futur proof?