Add aria-label and alt support

Original PR was eaten by git :\

This adds a `role` to core-icon and allows the developer to apply either `aria-label` or `alt` attributes to support screen readers. As a last ditch fallback it will try to include the icon name if one was provided.

cc @alice
The tricky bit is that both `icon` and `aria-label` may be altered by the user at any time.