state-based parser for multi-valued set selector syntax

Continuation of the work done [here](https://github.com/GoogleCloudPlatform/kubernetes/pull/951). Whereas previous work expanded the struct and went from object to string, this goes from string to object.
I don't feel strongly about it, but it might be more user-friendly to allow an arbitrary amount of whitespace instead of exactly one space (between all tokens, not just here).