New overlay utility

This closes #115. 

CodePen on Monday.
We do but they're applied via classes as it's a utility i.e. utilities can use other utilities as classes, only components can't. We have to `@extend` here as this is a pseudo element therefore there's nothing to hook the class onto.

Non-hover version HTML when it's not part of a component:

    <div class="u-position-relative">
      <div class="u-overlay-dark u-position-absolute u-position-pin-ends"></div>
    </div>