[12.x] Add database dump with data + load

[barryvdh]

The code is mostly in place, but we only dump schema now. We could extend this to include data, so we can just do php artisan db:dump and dump the entire database. Optionally we could then create db:load <file> to quickly import an entire file.

Status: work in progress. I think most drivers are similar, just an optional flag to include data or not.