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
`go-subscriptions` shouldn't have any specific awareness of the internal workings of `go-recurly`, so the change here would probably be to add a `get_meta()` method to this plugin.