only create dragondance.log when necessary

[vanhauser-thc]

I installed dragondance and every time I start Ghdira a dragondance.log file is created in the current directory. Over the time my file system is littered with these log files, although I use it only on few occasions.

How about only creating the logfile if an environment variable exists or the plugin is actually used?

[janbbeck]

I think that is a good idea. Can you make a PR?

[vanhauser-thc]

if I would a) code Java/Javascript and b) not swamped in a project and my own open source release I would :)