Fix unmarshaling of Parameter.Values

When using the extended [value syntax for parameter specification](https://github.com/apiaryio/api-blueprint/blob/master/API%20Blueprint%20Specification.md#description-11), the JSON that is generated for the Parameter.Values field is not a `[]string` but an array of further JSON objects (using [this sample template](https://github.com/danielgtaylor/aglio/blob/master/example.md)).

```json
{
  "name": "sort",
  "description": "Which field to sort by",
  "type": "string",
  "required": false,
  "default": "",
  "example": "joined",
  "values": [
    {
      "value": "name"
    },
    {
      "value": "joined"
    },
    {
      "value": "-joined"
    }
  ]
}
```