Shopkeeper4 integration

Snippet to display categories, products, filters, bread crumbs, etc. from the MongoDB database. For this extra to work, Shopkeeper 4 ( https://github.com/andchir/shopkeeper4 ) must be installed on your or remote server. For the shopping cart you need to use the snippet Shopping Cart ( https://github.com/andchir/modx-shopkeeper4/tree/master/core/components/shopping_cart ) (now in development).