Add an option to distribute shipping/promotions proportionally to items in order

[starsprung]

As discussed in #11

This is easy for single-item orders, but harder to do accurately for multi-item orders.

[couvares]

FWIW, I think that this is a great solution to the unsolvable problem of mapping coupons to items in a multi-item invoice (since I don't think Amazon provides enough information -- anywhere -- to figure it out programmatically, and thus something has to give).

Since it can't be done right without human input, I care less about discounts mapping perfectly to items in a multi-order than I do about reducing my data-entry burden (since that's the point of this tool in the first place).