Close Reveal modal with a button (.button.close-reveal-modal)

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: <a href="#" class="button close-reveal-modal">Close</a>

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.