Allow passing a node instead of a selector

I've come across some situations where I don't know how many sets of tab will be on a page, and so have to loop over all instances of `[data-tabs]` to initialize a Tabby for it. I've achieved this with..

```
const ELEMENTS = document.querySelectorAll("[data-tabs]");

if (ELEMENTS.length > 0) {
ELEMENTS.forEach((element) => {
new Tabby(`#${element.getAttribute("id")}`);
});
}
```

but that seems somewhat fragile and also confusing to read. Being able to directly pass `element` in this case would really simplify things.

Start at the Tabby constructor used in the issue's `new Tabby(...)` example and trace how its argument is initialized. Support passing the element directly while preserving selector usage, then verify that both forms initialize tabs correctly.

Written by the indexing model from the issue text.