Structured output

[smoehrle]

This PR is based on #225. Only this commit is new: 49c710e

It adds a flag --out-file which outputs some data in a structured format for further processing. Again, if you give me a hint, that this is something you would merge, I can further polish everything and make it available to others 😇

Example output:

$ ecs deploy app-messenger-cluster app-messenger-service app-web-cluster app-web-service  --out-file data.json [...]
$ cat data.json  | jq
[
  {
    "cluster": "app-messenger-cluster",
    "service": "app-messenger-service",
    "old_task_definition": "app-messenger-td:7",
    "old_task_definition_arn": "arn:aws:ecs:eu-central-1:111122223333:task-definition/app-messenger-td:7",
    "task_definition": "app-messenger-td:8",
    "task_definition_arn": "arn:aws:ecs:eu-central-1:111122223333:task-definition/app-messenger-td:8"
  },
  {
    "cluster": "app-web-cluster",
    "service": "app-web-service",
    "old_task_definition": "app-web-td:4",
    "old_task_definition_arn": "arn:aws:ecs:eu-central-1:111122223333:task-definition/app-web-td:4",
    "task_definition": "app-web-td:5",
    "task_definition_arn": "arn:aws:ecs:eu-central-1:111122223333:task-definition/app-web-td:5"
  }
]

[trigrab]

I rebased the branch and added a test.