Added functionality to declare config differneces as known (and thus ignored)

This functionality is useful to declutter the output of the difference from the conf tool. It eliminates from the output the noise because of intentionally different settings.

Also added a file that contains an example of such a differently configured component. @wlampl @goetz @emoyse Maybe you see some of the settings that should be added there immediately having a look at: http://tbold.web.cern.ch/tbold/diff-raw-14may.html