message: elle.log: honor ELLE_LOG_LEVEL in addition to ELLE_LOG_TARGETS
diff: @@ -174,8 +174,10 @@ namespace elle
  std::unique_lock<std::recursive_mutex> ulock{log_mutex()};
  if (!_logger())
  {
  - _logger() = std::make_unique<TextLogger>(std::cerr);
  - _logger() = make_logger(os::getenv("ELLE_LOG_TARGETS", "file://cerr"));
  + auto level = os::getenv("ELLE_LOG_LEVEL", "LOG"s);
  + _logger() = std::make_unique<TextLogger>(std::cerr, level);
  + _logger() = make_logger(os::getenv("ELLE_LOG_TARGETS",
  + "stderr://?" + level));
  }
  return *_logger();
  }
  