public static void init(int logLevel, Printer... printers) {
    init(new LogConfiguration.Builder().logLevel(logLevel).build(), printers);
  }
Initialize log system, should be called only once.

@param logLevel the log level, logs with a lower level than which would not be printed
@param printers the printers, each log would be printed by all of the printers
xlog/src/main/java/com/elvishew/xlog/XLog.java