Truncating based on config, not inserts

[clphillips]

It would be nice if invoking Fixture::down() truncated based on the tables defined by the config files, as opposed to the tables actually inserted into.

That would make it possible to use this library to seed databases. For example, I run my database migration, then I seed the database. The seed process needs to truncate data before inserting (which is contrary to fixtures for integration testing) because we're not starting with a fresh database.

Thoughts? I'm happy to hack on this and submit a PR.