Configurable shipping options and rates per vendor

[mariannk]

It would be good if each vendor can have vendor-based shipping methods (options) and buyers vendor-specific shipping options at checkout.

But considering that options from different vendors could be different and cannot be combined, then this feature will work only after we implement separate orders for vendors (split carts). It'll be available when we disable consolidated orders (and enable "split carts").

Let's decide what shipping plugin will support it (most probably "FixedByWeightByTotal"). And let's think who will configure it (a store owner only or vendors themselves)

Also see #56 (related)