Add support for event listeners

[michael-swan]

The XMLHttpRequest object is an EventTarget and therefore supports addEventListener. The event I am most interested in is "progress". Perhaps that event handler alone could be added as an argument to the affjax function.

[gasi]

FWIW, I found this Gist on how to report progress via lower-level Web.XHR module: https://gist.github.com/jmackie/b6afee6e1cb628ff5a600b626026aa5c#file-upload-purs