Support free shipping modes

[kassner]

Adding support to give free shipping via cart rules only to the cheapest method. This allows customers to pay extra if they want an expedited shipment option.

The change also allows developers to create their own free shipping mode by creating plugins to the FreeShippingMode::toOptionArray and Matrixrate::collectTotals methods, as the previous code was setting all prices as zero.

The configuration defaults to all, so it will have the same behavior as before.

[wsadasmit]

Thanks for the submission! We're not accepting new features for this extension currently, so I'm closing this, but we appreciate it!