Fix links without href

[lydell]

Closes elm/browser#34, closes elm/browser#55, closes elm/browser#64, closes elm/virtual-dom#142.

<a> elements without href aren’t clickable in plain HTML. However, in Browser.application programs, clicking them still produces a message.

The href attribute on a and area elements is not required; when those elements do not have href attributes they do not create hyperlinks.

— https://html.spec.whatwg.org/multipage/links.html#links-created-by-a-and-area-elements

If the a element has no href attribute, then the element represents a placeholder for where a link might otherwise have been placed, if it had been relevant, consisting of just the element's contents.

— https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element

Note to those following along: This PR is included in https://github.com/lydell/browser/tree/safe, which is part of https://github.com/lydell/elm-safe-virtual-dom, but it has nothing to do with “safe virtual DOM” really, it’s just in there because it’s convenient.