Support cdn field in walker

It simply goes through the file structure and attaches something like this to the output:

```json
        "cdn": {
            "1.2.1": [
                "colors.min.js"
            ],
            "1.2.2": [
                "colors.min.js"
            ],
            "1.2.3": [
                "colors.min.js"
            ],
            "1.2": [
                "colors.min.js"
            ]
        }
```