can we make this an `options` object instead, for future compatibility in case we want to add more. and then change the name from `plain` to `string`, so it would be like this. and then also add the `"templates"` key as the first argument to match the native `concat` and `copy` plugins

with all that it would end up looking like this:

```js
builder.use(jade('templates', { string: true }));
```