Explicitly set visited state of unstyled button

[andycochran]

Description

This PR explicitly sets the :visited state of unstyled buttons when they are also inverse and outline.

Note: This solves a problem that's specific to <a class="usa-button"> elements that include an href value (i.e. buttons that have a visited state).

When buttons have all three of these classes… 

.usa-button--outline.usa-button--inverse.usa-button--unstyled

…their ruleset includes the button-unstyled mixin, which in turn includes the typeset-link mixin, which makes the button look like an anchor by setting all 4 anchor pseudo classes (including a purple :visited state). So, when these 3 classes are used on a dark background, the :visited state is not inversed and the color is not accessible.

This proposed change overrides the mixin's :visited state with the inverse color so that an outline, inverse, unstyled button has the inverse text color and is accessible on a dark background.

[thisisdano]

Excellent — thank you!