Adds async attr to iframes, with a fallback

The `async` attribute is now supported well enough with a `defer` fallback that it’s safe to use. This is the perfect use case, as it’s for modules that

> Can be placed anywhere, ideal for scripts that can tolerate out-of-order execution.
> —Ilya Grigorik

More info on that in [this post on the `async` attribute](https://www.igvita.com/2014/05/20/script-injected-async-scripts-considered-harmful/). Essentially, this pull request (quickly, a little hack-ily) adds the `async` and `defer` attributes to every `iframe`, speeding up a page’s perceived performance in the browsers that support them.

Let me know if you have any questions about it. :)