property KeepExistingCounters added PerformanceCounters task

Hi Mike,
the pull request just contains a new property KeepExistingCounters of class MSBuild.ExtensionPack.Computer.PerformanceCounters. When this property is set to "true" then users can add counters of the given category incrementally. This can be useful in msbuild scripts which frequently add (and remove) performance counters and it may allow the code to be better structured. Actually this is something I would use in our msbuild scripts if that would be in the msbuildextensionpack library.
But it is indeed pretty simple, so please consider that as such, indeed:-). 

Cheers, Jan