Implemented command-line control of log level

Change-Id: I9d03c0a67856710f8ade5d98cdcdf26dd4277275
diff --git a/src/util/logging.cpp b/src/util/logging.cpp
index b067c80..d26b11d 100644
--- a/src/util/logging.cpp
+++ b/src/util/logging.cpp
@@ -42,11 +42,13 @@
 namespace ndn {
 namespace ntorrent {
 
+log::severity_level LoggingUtil::severity_threshold = log::info;
+
 void LoggingUtil::init()
 {
   logging::core::get()->set_filter
   (
-      logging::trivial::severity >= SEVERITY_THRESHOLD
+      logging::trivial::severity >= severity_threshold
   );
 
   logging::add_file_log