Allow active log file to be located in a separated directory

[Sauci]

In some case (i.e. embedded software applications), write operations to the flash might be an issue. Thus, it would make sense to add an additional parameter to the Logger structure (I would propose BackupDirectory), which specifies where the old log files should be stored. In this way, the active log file path can point to a RAM location of the file system (e.g. /var/log/... on Linux), while the backup directory can point to a flash location.