Second pass at handling post meta

Based on #168.

Still not at a point where I like it. We're now outputting:
```json
[
    {
        "key": "my_test_field",
        "value": [
            "Some value!"
        ]
    }
]
```

Whereas the input is:
```json
[
    {
        "key": "my_test_field",
        "value": "Some value!",
        "action": "update"
    }
]
```