Scroll to heading specified in URL

[lambdacasserole]

Fixes issue #7. I'm not sure how clean it feels to do it this way, but essentially we're simulating a click on the appropriate heading listed at the left hand side of the page in the menu, if the URL specifies a heading to scroll to (like this https://pages-themes.github.io/leap-day/#heading-2). This has a couple of advantages:

• As far as I know there's no way to do this without JavaScript
• It's decoupled from whichever scrolling framework powers the click event on the left-hand menu

But has a caveat:

• This will only work for headings listed in the left-hand menu (h1 and h2) so h3-h6 will have the same issue as before (unless we add them to the menu by changing the selector here).

I've tried to stick with the existing coding style.

[stale[bot]]

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