Allow ContentFieldsAPIResponse.Data to hold nested objects

By using a generic `interface{}` instead of a `string` ContentFieldsAPIResponse
type can be used to unmarshall ContentFields with nested objects, not
just strings.

For example, this kind of response would throw an error before:
```json
{
  "data": [
    {
      "name": "some name",
      "complex-object": {
         "content": "content"
      }
    }
  ]
}
```

api.go

@@ -72,5 +72,5 @@ type FeedAPIResponse struct {
 
 // Content Fields
 type ContentFieldsAPIResponse struct {
-	Data map[string]string `json:"data"`
+	Data map[string]interface{} `json:"data"`
 }