v4-alpha Card columns on small screens

Created by: James1345

Using the card columns forces 3 columns to be created regardless of screen size. On smaller screens (e.g. xs, sm) this is not particularly useful. It would be desirable to be able to set different column numbers for different screen sizes easily without manually overriding the column-count property

Syntax example - matching the grid classes. This is merely my own personal proposal card-columns-sm-1, card-columns-md-3