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/IXSocket.h

@@ -69,6 +69,7 @@ namespace ix
         std::pair<bool, std::string> readLine(const CancellationRequest& isCancellationRequested);
         std::pair<bool, std::string> readBytes(size_t length,
                                                const OnProgressCallback& onProgressCallback,
+                                               const OnChunkCallback& onChunkCallback,
                                                const CancellationRequest& isCancellationRequested);
 
         static int getErrno();