Support "disabled" links

[tesk9]

Adds "disabled" link support by following the recommendations in Scott O'Hara's "Disabling a link" post.

Essentially, when the link is disabled:

• removes the href
• adds role link
• adds aria-disabled true

Caveats

The styleguide example will NOT work correctly because of elm/browser#34, which describes a problem where "a tags without href generate a navigation event".

However, I still expect this to work in the NoRedInk context in most cases, including the case that's the reason that we're adding "disabled" link support at all.

[omnibs]

The changes look good. The only request I'd have before a merge is, if you're game, adding the caveat and link to the elm/browser issue in the disabled function docs, so it's easy to find if anyone gets confused in the future.

[tesk9]

Great idea! Doing it! Thanks