Freebird: the new go-recurly plugin

mostly pulled out of `go-subscriptions` and `go-subscriptions-freebies`.

see https://github.com/GigaOM/legacy-pro/issues/4125
we can call `esc_html` on `$key`, but `$vallue` can contain html (href link to user's Recurly account, for example). `wp_kses` on `$value`?