added optional "recurse" param to prevent recursion on subdirectories

Recursion down directory hierarchies should be the default behavior, but should be preventable if someone only wants the top level directories to be added to the hash. There's not an easy way via the `check` or `callback` params currently, so I added another parameter. Might be cleaner to pull the middle params (path, blacklist, recurse) into an options object at this point, but I didn't go that far.