Fix id starting with number

[acekat]

Hey !

This PR is to fix cases where ids are set and start with a number (this is valid HTML5).
querySelector raise a SyntaxError as it expects a valid CSS selector, yet we can use getElementById as we are only looking for ids

I changed the test fixtures to have 3rd-heanding as id and fixed the selector in src/util/updateActiveNavItem.js and src/util/updatePositionData.js

My demo Pen works as expected : https://codepen.io/acekat/pen/pooxLjy

Thanks for your work 👍

[jimmynotjim]

Thanks for this PR @acekat, sorry it took me so long to reply. Just a couple of comments and then I can merge this and set up a release.