Add option to generate only the summary from processed test results files

[workgroupengineering]

fixes: #122

[dorny]

Thanks for the PR and sorry for my slow response. Unfortunately, I'm quite overwhelmed with other work. I will try to look at it this week.

[dorny]

Thanks again :)
I've merged it with only a small modification - it forces generation of summary when there is only one test results file and the only-summary option is enabled. Otherwise, the generated report would be emtpy.