Added new attributes and some aliases to the {cart} substitution

[roadster31]

This PR is add attributes to the {cart} substitution

1. A new weight attribute is added, to get the cart total weight.
2. A new total_price_without_discount attribute is added, to get the cart total amount without taxes, excluding discount.
3. The following aliases of existing attributes are added, to provide a better english syntax, or a more accurate name :
   • product_count, alias of count_product
   • item_count, alias of count_item
   • total_price_with_discount alias of total_price
   • total_taxed_price_with_discount alias of total_taxed_price
   • contains_virtual_product alias of is_virtual
   • total_tax_amount alias of total_vat

[bibich]

Can you just squash your commits and I will merge your PR.
Thanks.

[roadster31]

Done !

There is a related PR in the Thelia doc : thelia/thelia.github.io#172

[bibich]

Thanks