feat(amazon): handle invoices with no per-shipment details

[jktomer]

Amazon has recently changed their invoice format to remove the per-shipment
totals tables (see issue #26 ). This change loosens parsing to handle such
invoices, without breaking parsing of older invoices.

Pre-tax adjustments and sales tax, if not present in the shipment tables, are
added at the per-invoice level instead. If there is only one shipment in the
transaction, they continue to be grouped with the lone shipment; otherwise,
like unrecognized post-tax adjustments, each is ungrouped.

[coveralls]

Pull Request Test Coverage Report for Build 108

• 26 of 29 (89.66%) changed or added relevant lines in 2 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 66.257%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
beancount_import/source/amazon.py	10	11	90.91%
beancount_import/source/amazon_invoice.py	16	18	88.89%

Files with Coverage Reduction	New Missed Lines	%
beancount_import/source/amazon_invoice.py	4	85.62%

Totals
Change from base Build 107:	0.2%
Covered Lines:	8810
Relevant Lines:	12918

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💛 - Coveralls

[jbms]

Thanks for fixing this!