Fail if a `<label>` contains more than one focusable item.

[Closes #103]
Actually link is not a labelable element: https://html.spec.whatwg.org/multipage/forms.html#category-label - so we probably need a different selector for labelable elements.

Also, it's not against the spec to have more than one labelable element as a descendant of a `<label>`, so is there a relevant recommendation that this rule rule refers to? I'm interested to see what the wiki is going to say, basically...