Add an @extendPrefix property

The doc viewer breaks `@extend` links for paper elements which extend core elements, since the hashtag is hardcoded in the href attribute of the link (see [core-docs-page.html#L51](https://github.com/Polymer/core-doc-viewer/blob/master/elements/core-doc-page.html#L51)).

Adds an @extendPrefix property to allow linking between element categories.