Make it possible to pass custom selectors

Right now, the library expects button elements inside the menu. While correct (I think), it might be a bit restrictive. It would be nice to be able to pass a selector to query the menu items. Also, I think the default should rely on direct children rather than any arbitrary selector.

[KittyGiraudel]

Then I’d say the easiest is to enforce direct children to be buttons. But I could see how v2 of the script comes back on this decision and allows more flexibility on the DOM structure.

So the README.md needs to be very clear about the expected structure to close this issue.

[Heydon]

It’s kind of a convention for controlled elements to be associated by id (see <label for="id">, aria-describedby="id", aria-controls="id" etc.)

The fact that multiple same ids are well known to be poor form (and come out as a parse error) helps make sure the developer doesn’t screw up.