Adds async attr to iframes, with a fallback

[kennethormandy]

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. 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. :)

[Jcmarc1flores]

Can You hice me accesos please