Tagging: Add additional optional parameter to tag command for size of pull

[Tomas Rojo Hernandez]

Right now the number of products being tagged lives in settings.py, potentially overridable through local_settings.py.

We should add an additional and optional flag to the tag command that specifies the number of products, that would override both local_settings.py and settings.py.

Example usage:

./cli.py tagging tag --output brat --category dresses --limit 20

The above command will pull 20 products from the dresses category, tag them, and write the output in BRAT format.