Want support for `async` option

[Swivelgames]

We're running into issues at the moment, because we want to force a synchronous request via the XMLHttpRequest module if we're on the server-side for a particular scenario. The issue with this is that it does not seem like w3c-xmlhttprequest will allow for this. It is understandable in some respects, but in certain specific scenarios this may be required. It is in the W3C specification as well, which I was under the impression this library implemented.

Please add this feature. It is a must-have for our project.

Thanks!