Feature allow files in contao/extra section.

Implement feature #3.
We now support:
```json
"extra": {
    "contao": {
        "_note": "sources is always synchronized, " +
            "old files deleted, neu files added, " +
            "changed files overwritten",
        "sources": { ... },

        "_note": "files is only merged, " +
            "new files added, old files preserved" +
            "(neither deleted nor overwritten)",
        "files": { ... },

        "_note": "userfiles is merged like files above but" +
            "all destination pathes are relative to Contao uploadPath",
        "userfiles": { ... }
    }
}
```