Css module support is broken in 3.1.3

In 3.1.2 docs example works, imports in js return correct objects.

```js
postcss({
modules: true
})
```

In 3.1.3 same code imports return `undefined`. From https://github.com/egoist/rollup-plugin-postcss/pull/292 there is new `onlyModules` option I believe. Using it brings back correct objects.

```js
postcss({
modules: true,
onlyModules: false
})
```

However, it's a breaking change, this option is not documented and I have no idea if using it is a correct workaround.

Would be awesome to see official solution to this one 🙏