Add Filtering by Number of Days Since Node Modification

Attached is a patch to allow an admin to specify a number of days to go back looking for tags that have been applied to nodes. It will not be completely accurate, as it will capture the changed date of a node regardless of whether the changes to it have anything to do with tagging. However, it was a quick-and-dirty tool for what I need, so I submit it here that others may make use of it (or transform it into a more accurate solution).