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
not yet, and good point! i'll add the checks. not sure if we want to make these decencies "softer" by using filters or something like though. I was under the impression that it's ok for `go-recurly` to depend on `go-subscriptions` (but not the other way around).