Auto-expand by URL, highlight on scroll

[hallvors]

Hi, here are some suggested enhancements (two, perhaps I ought to make different PRs)

One change will mark a menu entry active when the page is loaded (and expand parents) if the link references the current address.

The second improves handling of document-internal #anchor links by setting aria-selected=true on such links when the content they refer to is scrolled into the viewport.

I hope you will consider these changes @longmatthewh !