Option to output CSS instead of JS

:wave: Hello! Thanks for publishing this useful package 😄

I'm the author of another CSS plugin for rollup, [`rollup-plugin-lit-css`](https://npm.im/rollup-plugin-lit-css), which is a simple plugin for importing css files into javascript as `lit-element` css tagged template literals.

I'd like for my users to have the option to pass their CSS through postcss before loading into JS, and have the postcss rollup plugin output a string of CSS without adding js syntax.

## Input
### app.js
```js
import style from './styles.css'
console.log(style)
```
### styles.css
```css
main {
background: red;
&.nested { background: blue; }
}
```
### rollup.config.js
```js
import postcss from 'rollup-plugin-postcss'
import litcss from 'rollup-plugin-lit-css'

export default {
input: 'app.js',
output: {
format: 'es',
file: 'bundle.js',
},
plugins: [
postcss({
output: 'css',
inject: false,
plugins: [
require('postcss-preset-env')({stage: 0})
]
}),
litcss(),
],
}
```

## Expected bundle.js
```js
import { css } from 'lit-element';

var style = css`main {
background: red
}
main.nested { background: blue; }
`;

console.log(style);
```

## Actual bundle.js
```js
import { css } from 'lit-element';

var style = css`var css = "main {\n background: red\n}\nmain.nested { background: blue; }\n";
export default css;`;

console.log(style);
```

If this option is already available, I wasn't able to find it in the docs or the source. From a brief tour of the source, it looks like the plugin always exports JS.

Thanks again for publishing this.