cannot specify filename with sketch.export()
Considering following code, who exports images which is referenced later, so I need specific names which is assigned by myself. But the `output` takes `option.output` as a directory.

```ts
export function export(layer: Layer, format: MyExportFormat, path: string, name: string) {
    let savePath = [
        path, "/",
        format.prefix, name, format.suffix,
        ".", format.format
    ].join("");
    // cannot use sketch.export because it's option.output is a directory, 
    // we don't have a chance to specify the filename,
    // the filename is unpredictable.
    sketch.export(layer,{
        output: savePath,
        formats: format.format,
        scales: format.scale.toString(),
    })
    return savePath;
}
```