#410 Artifact cleanup extension

This feature seems to need an exact path to exclude. I wonder whether it will be as useful as having a pattern. Would plugin authors want to delete all zip files in a stage, for instance? I don't know. I'd assume a more generic pattern based system is more useful. What do you think?

Could be done later too. If later, we could add an "excludePatterns" parameter to the request (not now).