nsq_to_file: allow path separator in filenames

Putting a path separater in filename-format or datetime-format will
cause nsq_to_file to create and fill a directory structure. Useful for
e.g. dumping files in a tree that is easy to import to data warehouse
tools.
Merge pull request #319 from jsocol/nsq_to_file_create_paths

nsq_to_file: allow path separator in filenames