add logging struct with -u json options flag, log tiling progress [#13]

json_logger.cpp

@@ -0,0 +1,8 @@
+#include <stdarg.h>
+#include <fstream>
+
+#include "json_logger.hpp"
+
+void json_logger::progress_tile(double progress) {
+  fprintf(stderr,"{\"progress\":%3.1f}\n",progress);
+}