[WIP] Add accordion elements

[shawnbot]

I started this as a Custom Element implementation, but then decided against it and went with a variation on this practical ARIA example. It expects HTML in the form:

<div class="picc-accordion">
  <h1>
    <a aria-expanded="false" aria-controls="accordion-content">
      Click me to expand
    </a>
  </h1>
  <div id="accordion-content" aria-hidden="true">
    <p>This content will be hidden by default, and shown when the heading is clicked.</p>
  </div>
</div>

Hey @nickbristow, can you tell me if it's okay to use <a> instead of <button> for the clickable heading? Do I need to use aria-role="button" if it's an anchor, or add tabindex="0" in either case? The anchor implementation doesn't appear to allow use of the keyboard to "click" it.