Option to dump table rows as individual files in a directory

[simonw]

Inspired by the Who's On First directory layout:

https://github.com/whosonfirst-data/whosonfirst-data-admin-us/tree/master/data

I'll have an option for "dump each row as a file in a directory named after the table" and a separate option for "and use 123/345 style subdirectories when you do it".