make visually-hidden extend a placeholder

- Add a placeholder called "visually-hidden" that contains the styles that were
  previously in the mixin "visually-hidden"
- Modify the "visually-hidden" mixin to merely extend the new "visually-hidden"
  placeholder.

Since visually-hidden is a reusable style that never changes, it should be
harnessed as an extended placeholder to prevent it from being duplicated
unnecessarily.

[This is the way](https://github.com/Snugug/toolkit/blob/master/compass/stylesheets/toolkit/_clearfix.scss) @Snugug often approaches this, and I like it. What do you think?