allows polyfill to be installed at 'interactive' document ready state

In some cases document can stuck in 'interactive' state which means document is finished parsing but some resources are still being loaded https://developer.mozilla.org/en-US/docs/Web/API/document.readyState

Supporting this situation is important since there are cases where scripts are loaded dynamically so you can't rely on DOMContentLoaded event since it could be already fired
http://mail-archives.apache.org/mod_mbox/incubator-callback-commits/201206.mbox/%3C20120601235227.4EAE5F656@tyr.zones.apache.org%3E