Add Paid Memberships Pro support

[rvdsteege]

The Paid Memberships Pro plugin used to be on our list of rejected feature requests at https://github.com/pronamic/wp-pronamic-pay/wiki/Rejected-Feature-Requests, because developers could not add their own payment gateway. In a later update it became possible to add payment gateways, which was already mentioned in the rejected feature requests:

Update 2016-08-04 - Since version 1.8 it is possible to add new gateways:
http://www.paidmembershipspro.com/gateway/adding-a-new-gateway/.

Therefore, I'm opening this discussion to track feature requests from users for integration with Paid Memberships Pro. We've now received such a request in https://wordpress.org/support/topic/paid-memberships-pro-17/.