--output=json flag to allow external tooling to be developed on top of cptest

[kuredoro]

cptest produces go test -v-like reports, but if somebody wants to process the reports themselves (to generate html pages, etc.), they have no pain-less way to do it. This proposal aims to increase interoperability with other tools.

The format needs to be decided. It should interoperate with #6 too. This is minimal, I think

[
    {
        "input": "foo\n",
        "expected": "foo\n",
        "stdout": "bar\n",
        "stderr": "whaaa?\n",
        "exit_code": 0,
        "run_time": "1.04s",
        "verdict": "TL"
    },
    null,
    {
        // ...
    }
]

[kuredoro]

So, okay, ALL of cptest output should be in json. Meaning that errors should go there too. Moreover, a struct is a better choice anyway, since it can be extended without breaking backwards compatibility.

v0.2

{
    "results": [
        {
            "input": "foo\n",
            "expected": "foo\n",
            "stdout": "bar\n",
            "stderr": "whaaa?\n",
            "exit_code": 0,
            "run_time": "1.04s",
            "verdict": "TL"
        },
        null,
        {
            // ...
        }
    ],
    "errors": [
        {
            // think about how to represent the error hierachy
        }
    ]    
}