Create copies of multiple API backends/products?

[smccarthy-ie]

The new backend copy (#233) and product copy (#235) commands are currently limited to copying a single backend/product at a time. Propose updating both commands to enable creating copies of multiple backends/products?

For example:
3scale backend copy -s 3scale1 -d 3scale2 my_api_backend_01 my_api_backend_03 ...
3scale product copy -s 3scale1 -d 3scale2 my_api_product_01 my_api_product_02 ...

Note: This is a question I had when documenting these features. We would need feedback from PM on whether this is useful/required by customers.