Add support for tags

[cgwalters]

Currently the model is that builds.json implements a linear stream of builds, a lot like ostree/git commits. But we should add the concept of tagging.

See prior discussion in e.g. #177

What I'm thinking here is that builds.json would look like:

{ "builds": ["42.4", "42.3", "42.2"]
  "tags": [{"name": "smoketested",
               "created": "2018-09-26T15:51:42Z",
               "target": "42.3"}, ...]
}

Created by e.g.: coreos-assembler build-tag create smoketested 42.3

Our pruning logic would be enhanced to understand these additional references.

[wking]

Cross-linking openshift/os#351.

[jlebon]

PR for the tag command in #243.

[jlebon]

PR for teaching prune about tags in #245.

[cgwalters]

I think we can call this fixed.