Allow passing on XHRUpload options, such as "limit" to AwsS3 Plugin

[ogtfaber]

I need this option to keep large amount of uploads working: #468

[goto-bus-stop]

thanks! i worry that a xhrOptions option makes the abstraction a bit leaky--it would be nice if the user never had to know that XHRUpload is being used internally. (and in the future, we might teach the AwsS3 plugin to use 'multipart' uploads for large files, which wouldn't be able to defer to the XHRUpload plugin). perhaps we could take a limit option like .use(AwsS3, { limit: 10 }) instead and just pass that through? what's your thoughts on that?

[ogtfaber]

@goto-bus-stop that makes sense. There's still more work to do since (as you know ;-) the "limit" option does not work well with the plugins currently.

[ogtfaber]

@goto-bus-stop how about this

[goto-bus-stop]

yeah this is perfect, thanks! good call on adding timeout too. we'll have to address the problems with limit in the new year i suppose :)

[ogtfaber]

Awesome, thanks @goto-bus-stop