Find existing products before creating new ones

- added -m argument to product_images.thor for attribute to match when finding existing products
- modified product_loader to override perform_csv_load
- if match_by argument is provided, use find_or_new to look for an existing product
- added current row and name to command line output
- improved error message when number of variants doesn't match number of variant attributes

Note that this doesn't handle finding existing variants. If you run the script twice, a product with three variants will end up with six.