Provide means to store 'system artifacts' - that can be configured by plugins

To avoid the need for a user to configure the location of the clover.xml (and the clover HTML too), allow a new mechanism for storing build artifacts that are available to the 'system' only. ie users should not be able to edit, add or remove them.

The proposed 'one-click' Clover integration option, would use this feature so a user can simply check a checkbox to have code coverage reported for their build, without a user having to specify its location.