Add 'cleanup mark/unmark' actions

[theCalcaholic]

I'd like to propose a feature for the cleanup command:

Additionally to cleanup delete and cleanup scan the actions cleanup mark and cleanup unmark could be added which save a timestamp for the entity if the (usual) conditions are met.

The marked status of an entity (and the age of the mark) could then be used as condition for the delete command (e.g. cleanup delete markedlongerthan 5d).

An alternative to the mark/unmark syntax is to provide a mark_or_keep_marked action instead.