Added screen reader only mixin

[firstchair]

This mixin hides an element to all devices except screen readers.
Naming up for debate, could be @mixin screen-reader-only(); or @mixin visually-hidden();

[ReneDrie]

I think the mixin is good, but I'm wondering wether it should be a mixin. To avoid repetition I'd rather make this a utility class (which can't live in this library atm). So it might make more sense to add a .sr-hidden class on the _base.scss of our projects for instance.

[firstchair]

@ReneDrie Indeed, this could be a global class to have less code but this is the same for the image replacement mixin for example. We could also have some default classes that we import.