"Native Checkout" method support

[manospsyx]

Ref: https://github.com/VivaPayments/API/wiki/Native-Checkout

https://support.vivawallet.com/hc/en-us/articles/201895281-What-is-the-difference-between-Redirect-and-Native-Checkout-

Native Checkout allows you to create a custom payment form, so that your customers never leave your page for making a card payment. This process is fully PCI-DSS compliant, as sensitive card data never reach your server.

Native Checkout, only allows you to accept card payments from a customer. You are therefore missing out on the Cash, Pay@Home and VivaWallet alternatives. To give additional payment options to your customers, we recommend using the ‘Redirect’ method instead.

Native Checkout seems to support payment tokens. Since WC 2.6 added support for tokens, this could be a very welcome feature for shop admins who don't care about missing out on the Cash, Pay@Home and VivaWallet payments (available with Redirect Checkout method only).

@mattyza a major feature to keep in mind for v1.1, possibly along with #5 .

[tcrosswell]

It would be great to see native checkout