Accordion - prop for heading level

[jgiardino]

A new prop headingLevel should be added to the Accordion component to allow the user to specify heading levels. Supported values should be h1 – h6.

Note: prop name and values are consistent with Title component.

[jgiardino]

Some extra notes on this:

1. As an example, if I set headingLevel="h2", I should see the following results in the html:

   <dl class="pf-c-accordion">
     <dt>
       <h2>
         <button id="ex-toggle1" class="pf-c-accordion__toggle" variant="plain" aria-expanded="false">
           ...
         </button>
       </h2>
     </dt>
     <dd>...</dd>
   </dl>
   

   Whatever value I specify for headingLevel should be used for the heading element that wraps the button.

2. My best guess is that this should get applied to the parent component, and applied to all the headings in that component. For example:
   <Accordion headingLevel="h2">
   My reason for this is that these headings are all peers and therefore should have the same heading level. And it simplifies setting this value to only have to define it once. If the use case exists to have different heading levels, then really the right structure for this type of heading outline is nested accordions.

3. The only other component where we have a headingLevel prop (that I have found) is the Title component. This component uses "h1" as a default if headingLevel is not defined. I'd say let's go with that for consistency.