Allow file naming via project.json

[jneug]

Jinjas syntax might cause problems on some filesystems that don't permit the required special characters.

A possible solution would be to allow renaming via a special field in project.json.

For example:

{
    "filename.txt": {
        "type": "rename",
        "value": "{{ time '%Y' }}.md"
    }
}

[jneug]

Mixing with fields makes no sense. Rather introduce separate sections in the config:

{
    "fields": {
        ...
    },
    "files": {
        ...
    }
}

[jneug]

See 453868aa0fc641d1aede5dd7ae1ecb1a9e5a402a