Allow the pubsub implementation to be configurable

[tom--pollard]

The pubsub library gocloud.dev/pubsub has various 'drivers' (for instance the gcppubsub package) which makes it relatively easy to interact with different 'pubsub' type systems via a common api.

package-feeds has used this extensibility to add support for Kafka, whilst keeping the specific differences behind a generic caller interface.

[calebbrown]

I believe this is done.