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
i'd love to reduce nesting but there's an `elseif` below this so we'll need a level of nesting even if we return here after checking both the `if` and `elseif` conditions fail. but i'll definitely take out the redundant `isset()`.