Close a Reveal modal with a button by adding the "close-reveal-modal" class (no additional javascript required)

Allows you to assign the .close-reveal-modal class to a .button within a
.reveal-modal, allowing the user to close the modal by clicking the
button.

Example: 
```html
<div id="myModal" class="reveal-modal">
    <a class="close-reveal-modal">&#215;</a>
    <p>
        Click the button below to close me.
    </p>
    <a href="#" class="button close-reveal-modal">Close</a>
</div>
```

Utilizes the CSS :not() selector to ensure that the styles applied to
the close "x" displayed in the top right (or left depending on your
$defaultOpposite variable) corner of the modal are not applied to the
button.