Add decorator attribute for methods to skip them in stacktrace

Currently everything in the StackTrace above type LogImpl will be concidered relevant.

I build a helper class that I don't need to see in every log entrys %stackdetails.

Also %type and %method always show the helper, of course.

It would be nice if log4net would define an attribute that I can put on that method to ignore it in the StackTrace.

Should be dead simple. 

It might have an impact on performance. So it should be an Opt-In-Feature.