Add pluralize filter from github.com/bdesham/pluralize
Add comment with attribution and license for pluralize.rb.
Store prevailing rates for each unit; hoist logic to Invoice::Processor.
Make sure an unspecified Line unit is nil when it reaches Liquid.
Add start_date and end_date to Line.
Support syntactic sugar for Lines on a single day.
Convert dates provided as ISO 8601 strings.
Expose invoice totals to Liquid.
Add test case for full conversion.

This triggers a bug when Jekyll performs its default excerpt extraction
which results in an invoice with no lines.
Ensure net_total and tax return numbers when no lines are added.