Added xhr readystatechange handler to options

[ludeus]

I need to add a listener on xhr, a quick fix would be to add an option

[AidasK]

I think this handler is missing some params, like which chunk or file has fired it?

[AidasK]

Please make it camelCase and add it to the docs

[ludeus]

I added the xhr option, inspired by the jquery ajax xhr option (http://api.jquery.com/jquery.ajax/). An additional parameter gives the file object

[drzraf]

This can be already be worked around by hijacking uploadMethod:

uploadMethod = (uploadMethod, fileObj, chunk) => { chunk.xhr.onreadystatechange = () => 'foo'; return 'POST'; }

[AidasK]

uploadMethod looks like a workaround, so I think this could be merged

[ludeus]

uploadMethod is a good solution