ofUrlFileLoader progress function

[NickHardeman]

It's rather hidden, but super handy.

ofURLFileLoader floader;
ofHttpRequest req;
req.progressCallback = [](const ofHttpRequest & req, float progress) {
	ofLogNotice("ofApp") << "http progress: " << progress;
};
req.url = "https://openframeworks.cc/about/0.jpg";
req.method = ofHttpRequest::GET;
req.timeoutSeconds = 60;
ofHttpResponse res = floader.handleRequest(req);
if (res.status != 200) {
	ofLogError("ofApp") << "Image download failed";
}