public Builder throwableFormatter(ThrowableFormatter throwableFormatter) {
      this.throwableFormatter = throwableFormatter;
      return this;
    }
Set the throwable formatter when {@link Logger} log a message with throwable.

@param throwableFormatter the throwable formatter when {@link Logger} log a message with
                          throwable
@return the builder
xlog/src/main/java/com/elvishew/xlog/Logger.java