fix: make inspect print JSON

[technosophos]

Unwrap any signed content

$ duffle inspect helloworld:0.1.1 --insecure
{
    "name": "helloworld",
    "version": "0.1.1",
    "description": "",
    "invocationImages": [
        {
            "imageType": "docker",
            "image": "cnab/helloworld:0.1.1"
        }
    ],
    "images": [],
    "parameters": {
        "port": {
            "type": "int",
            "defaultValue": 8080,
            "required": false,
            "metadata": {},
            "destination": null
        }
    },
    "credentials": null
}

Closes #336