Add Proper ARIA Roles

[srsgores]

This addon should really use the role="alertdialog" assertive role for screen readers.

All we have to do is add corresponding attributeBindings and role: 'alertdialog' on the component.

Also, as the document specifies, we should also add:

• aria-labelledby
• aria-describedby

As top-level attributes on the modal.

Finally, I might advise adding a role="button" for non-button close elements... but I digress that that might be something left up to the user.

[lukemelia]

@srsgores Sounds good. We would gladly accept a PR implementing these suggestions.