Card and its variants: outline variants and the need for .card-inverse

Created by: petetnt

I am a huge fan of the new Card-component. Like many other components, it also has some variants. These apply for the background only, and require another class, .card-inverse for proper text colors.

My suggestions:

• Add the .card-inverse styles for the .card-primary, .card-info, .card-danger, .card-warning and .card-success classes as an mixin
• Add new classes .card-primary-outline, .card-info-outline, .card-danger-outline, .card-warning-outline and .card-success-outline which would style the borders only, otherwise maintaining the regular style similar to those on .btn's for quick styling