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.

docs/usage.md

@@ -520,11 +520,21 @@ bool async = true;
 HttpClient httpClient(async);
 auto args = httpClient.createRequest(url, HttpClient::kGet);
 
+// If you define a chunk callback it will be called repeteadly with the
+// incoming data. This allows to process data on the go or write it to disk
+// instead of accumulating the data in memory.
+args.onChunkCallback = [](const std::string& data)
+{
+    // process data
+};
+
 // Push the request to a queue,
 bool ok = httpClient.performRequest(args, [](const HttpResponsePtr& response)
     {
         // This callback execute in a background thread. Make sure you uses appropriate protection such as mutex
         auto statusCode = response->statusCode; // acess results
+
+        // response->body is empty if onChunkCallback was used
     }
 );