[WAT] Add rel attribute to A@/href links

The `rel` attribute isn't extracted for  A (and AREA) hyperlinks. The [link types](https://html.spec.whatwg.org/multipage/semantics.html#linkTypes) specified are useful, e.g., `nofollow`. Also check whether other [attributes](https://html.spec.whatwg.org/multipage/semantics.html#links-created-by-a-and-area-elements) are worth to be extracted.