chaining Steps together

[frantjc]

some sequence steps such as concourse resources and the action-git-cloning portion of a uses step need to relay information back to sequence.

the uses step specifically will need to do this to dynamically run containers depending on what it finds. to achieve this, the step will emit json on stdout that looks something like

{
  "step": {
    "image": "alpine",
    "entrypoint": [...],
    "cmd": [...]
  }
}

sequence's runtime.Run functions should anticipate which containers it runs will do this and parse stdout from such containers, returning the parsed response. sequence should then check the response for a step and, if its present, run that step immediately