Honour fragment link on nav and hashchange

[hidde]

I've added some code that opens section content when page opens with hash or when the hash changes (fixes #3).

This works both if:

• heading that is the label is the fragment
• content that 'belongs to' heading (eg tab panel, collapsible content) contains the fragment

Demo here: https://codepen.io/hidde/pen/RwZbgVK

One weird thing: in Firefox, the location.hash change only works if wrapped in setTimeout (1 millisecond works 🤷🏻‍♂️); in Chromium this hack is not required.