Add inverse mixin and modifier for links

[36degrees]

Links on dark backgrounds (like blue 'heros') are generally white.

Because of the way that the different states interact with each other, and the importance of the ordering of the selectors in the CSS, it's relatively easy to end up with the focus state being incorrectly applied – for example, with focused visited links being white on yellow:

We should consider adding an govuk-link-style-inverse mixin and an govuk-link--inverse modifier for links that sets the text colour to white and correctly applies the different states.