Adds a variable for the background color of the active tab in the tab component.

Right now, i have to override it like this:

```scss
.tabs {
  dd {
    &.active a {
      background-color: #EC7B4F;
    }
  }
}
```

This would make it much easier to specify the color in my `foundation_and_overrides.scss` file for my rails project.