Include a method by which to exclude directories from recursive sub lots

[jhiemstrawisc]

If a lot path is set to /foo and recursive=true, that indicates that everything under /foo belongs to the lot. However, we may wish that /foo/bar is explicitly excluded from in statistics calculations, but there's currently no way to realize this without explicitly adding every in-path. That's overly burdensome, so we should add an excluded-paths key to the lot database.