Express Checkout - Billing Agreements

[angelleye]

#105 will expand on this, but for now we can simply add the billing agreement parameters to the SetExpressCheckout request so that at least the billing agreement gets setup and then people can process new transactions through the details in PayPal using the "Create a Transaction" link.

That link takes people to a form where they can process a new amount using the billing agreement:

When the process a payment this way it won't end up in WooCommerce order history, of course, but at least they can process reference transactions manually through PayPal.

[angelleye]

We would need an option to enable/disable this feature, of course.

[jigneshkailam]

I am done with this and push to issue branch for your review.