Please add option for variations (New feature)

[ufoloko]

Some other shopping cart modules, in other systems (opencart, shopify, etc), have an option for variations.

For example, you have for a product(shirt) an option with colors (blue, black, white), but additionally you have some sizes for each color (s,m,l), so a variation can be shirt blue in small, with his own price and ref code, other shirt can be red in large, etc.

This is a new feature, but is the only feature i think opencart needs to be the most complete shopping cart solution.