Support custom backup directory and time format

[yinonavraham]

Add support for customizing the logger with the following features of backup (rotated) log files:

• TimeFormat - set a custom time format to use for the timestamp in the backup file name: foo-<timestamp>.log. If not set, the default time format is used, as was done until now, to keep backward compatibility.
• BackupDir - set a custom directory location where the backup log files shall be saved to when the log is rotated. If not set, the backup files are saved in the same directory as the active log file, as was done until now, to keep backward compatibility.