New Cartitem Data Filter

Added a filter ‘shopp_cartitem_data’ to filter the data array that’s
before it’s added to the cart. This allows me to filter out inputs with
empty data.
Fixed foreach loop

Went with @jonathandavis suggestion of combining it into one line. Derp.
Merge pull request #2899 from ingenesis/iss2896

Added shopp_cartitem_data filter hook