Several features require selecting sets of resources through selectors. See #199 for one example. We may want to use this for networking, volume selection and even load balancing. We need a common Selector type for use across the API to express this.

A few common requirements:

1. Match over attributes, types, names, and labels, fully or partially.
2. Additive selectors describe an intersection.

Whatever we choose should be compatible with docker's system.

1. Docker's engine filters: https://godoc.org/github.com/docker/engine-api/types/filters
2. Swarm's constraint filters: https://docs.docker.com/swarm/scheduler/filter/

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