[paymentrequest] Price breakdown (#25)

Most merchants break down their final price into separate line items, such as subtotal, sales tax, shipping cost, total, etc. Currently, the spec only requests one number, `amount`.

For what it's worth, Apple Pay uses an [array of summary items](https://developer.apple.com/library/ios/ApplePay_Guide/CreateRequest.html#//apple_ref/doc/uid/TP40014764-CH3-SW1), where the developer is responsible for totaling up the final price. The shipping method is treated as a [special summary item](https://developer.apple.com/library/ios/ApplePay_Guide/CreateRequest.html#//apple_ref/doc/uid/TP40014764-CH3-SW5), and the last summary item is reserved for the grand total.

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/paymentrequest/issues/25