Add --data-only to copy data without structure

[reidab]

This was prompted as a workaround for the issue presented in #139.

By allowing data-only transfers to be run, we can use multiple config files in order to extract multiple subsets from the same table. An initial run, without --data-only, copies the DB structure and anonmized users while a second run using --data-only and a different config file copies unmodified versions of admin users to the same table.

It's likely useful in many other cases as well.

I'm happy to add some testing for this if necessary, but I'd appreciate some pointers on where best to add it. There doesn't seem to be any existing tests for the Dumper Engine or the steal command, where the bulk of this takes place.