Fix `ReferenceError: Element is not defined` in Node.js (#33)
Fix `ReferenceError: Element is not defined` in Node.js (#33)

Since there's no `Element` global in Node.js, it's not safe to assume
that `Element.prototype` exists. Looks like this broke in 3.0.0.

Tabbable isn't actually useful in server-side code, but universal React
components may still end up loading Tabbable via focus-trap-react when
rendering on the server.
focus-trap/tabbable,focus-trap/tabbable,davidtheclark/tabbable