Update main.js

This is a size improvement over repeated document.querySelector calls, aiming to better shrink once passed through minifiers.
The script is small already but being a base reference, why not smaller, if it's possible keeping it clean?
> That being said, I have to rebase and receive confirmation I can sign something ^_^

:cake: Looking forward to it. We're undoubtedly going to run into disagreements over how to approach problems. In this case, I generally use `var $ = document.querySelector.bind(document);` in my own projects too but agree with you that for the audience we're targeting it's likely to introduce confusion. 

I'm fine with `querySelector` once we're able to merge in your changes.