Use different and proprietary log file extension

[frantisekbrabec]

Currently, the logs are created with the extension .log. This is very generic, may already associated with another app on any given system or may be undesirable to associate this extension with an osgar-specific utility. I would recommend the default extension to be switched to something unique, something like .olog perhaps.

[zbynekwinkler]

I am ready to go for it. I am working on some tools automating log analysis and it would be useful for me to be able to list all osgar log files in a directory... and .olog seems as good as any.