Add support for streaming transfers (#353)

This change adds onChunkCallback to the request. If defined it will be
called repeatedly with the incoming data. This allows to process data on
the go or write it to disk instead of accumulating the data in memory.

ixwebsocket/IXProgressCallback.h

@@ -7,8 +7,10 @@
 #pragma once
 
 #include <functional>
+#include <string>
 
 namespace ix
 {
     using OnProgressCallback = std::function<bool(int current, int total)>;
+    using OnChunkCallback = std::function<void(const std::string&)>;
 }