Feature/search across registries

This enables searching across all registered registries, but we only seed docker hub, so this *shouldn't* change anything for end users today. This may break the tags stuff if used with private registry, which I am tackling next.
Was determined not to use #is_a? here... If someone thought this a little to "clever" I'd be willing to compromise. Although, I think this is actually more reliable than checking in an if/else with is_a?