Add tariff option to `importCost`

[michaelrivet]

Why is this change happening?

I live in the US and want to use this library, but need to determine the impact new tariffs will have on my import costs.

This introduces a tariff rate option, which I can pass in config to determine what the import cost is when tariffs are applied. I see this as being an MVP version as I need to input the tariff rate manually. In the future I can introduce a new PR to read the package JSON and user settings to determine the tariff rate automatically.