Move API to be compatible with AsyncIterable spec

[jmorrell]

This updates the API to implement the AsyncIterable interface. It's a bit of a step back in terms of usability now, but conforms better to where the language is going. A standard interface will work better with third party libraries and the eventual changes to the language.

Eventually we'd like to be able to do this:

for await (const url of itemize(...)) {
  console.log(url)
}

Spec: https://github.com/tc39/proposal-async-iteration
Informational blog post: http://2ality.com/2016/10/asynchronous-iteration.html

[jmorrell]

Idea: replace current next method with nextUrl or nextItem to keep the aesthetic API for now, but still have a spec-conforming next method.