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
an alternative is maybe @okredo can work on this in parallel. the plugins are functioning now and this task is discreet enough that it can be done in parallel of `go-local-subscriptions`.