Update link-colors Sass mixin to use readable-color when hover color is not supplied

[chrisblakley]

Currently this is often used:

@include link-colors($primary_color, #fff);

But if the primary color is (for example) yellow, the white hover will not provide enough contrast ratio. Instead, I'd like to call them like this:

@include link-colors($primary_color);

Then in the mixin, the hover color will use readable-color($normal) (or whatever the syntax is) so that the contrast ratio is always preserved.

An example of this is inside of the _navigation.scss partial.

[chrisblakley]

This may actually be harder than it seems because we don't know (and can't assume) the background color...

The above code snippets assume that the background color changes to the $primary_color on hover which would work with the readable-color() hover, but if the background becomes $secondary_color on hover, then we don't know what color that is, so it wouldn't work in that case.