Make --rate sticky

[rouilj]

When invoicing by time, the rate is often fixed. E.G. --rate 175 dollars/hr. I would like to be able to use the following command line:

./invoice generate --due "net 30" --from "XYzzY Inc." --id "20230703-33" --title "Invoice for June 2023" --to "ABCD Services" --tax 0.06 --item "Addition of Autoresponder" --quantity 10  --rate 75 --item "Phone review of proposal" --quantity 1

Currently the "Phone review of proposal" is printed in the PDF with a rate of 0.00. I would like it to reuse the last rate defined.

Thanks.