Card header color is not themable with Sass variable

[andrewpucci]

Since the .card-header background-color is able to be themed using $card-cap-bg, the color should also be themable, otherwise you're stuck with the default color for whatever style text you put there. This could lead to low contrast between the background and text color, like so:

This would ideally be reconciled by adding a new variable, but could perhaps be fixed by using the color-yiq function to automatically set the color based on the value for $card-cap-bg.