Refactor core/baseline() to handle multiple targets.

[forestmonster]

When creating a baseline, additional targets are ignored (https://github.com/Netflix-Skunkworks/diffy/blob/master/diffy/core.py#L72). However, some TargetPlugin derivatives will submit multiple targets. We should be able to set a dictionary key in target_plugin["options"] indicating multiple targets, pass that using **kwargs, and add decision logic in core.py that acts on multiple or single targets based on the value.