Ability to add css class to card

[ArmeniaH]

I wanted to create something similar to this to skip tones of boilerplate, and I came across to this implementation, which is almost exactly what I needed.

Thanks for saving my time :)

It would be very useful, if we could also add css class to parent container of the card to have ability to write custom css for cards.

->class('some-class-name')

and in Card.vue

cardClassList() {
    return this.card.class + ' ' + this.cardCenter + ' ' + this.textClassList;
},

I am missing my working laptop, so I am sorry for not creating a pull request for this. Thanks a million !