Support for request progress monitoring

[p-himik]

I guess it's not that hard to implement given that request->xhrio-options already creates goog.net.XhrIo that supports setProgressEventsEnabled.

[kennethkalmer]

Providing a :progress-handler key works:

{:http-xhrio {:progress-handler #(dispatch [::upload-progress %])
              ; ...
              }}

But this very much tied to request->xhrio-options here, and goog.net.XhrIo according to the cljs-ajax README.