ES6 Export Not Compatible with ESLint 4.0?

Thanks for this library!

I tried using this with `eslint 4.2.1` with this config:

```json
{
"env": {
"browser": true,
"commonjs": true
},
"extends": [
"eslint:recommended"
],
"plugins": [
"eslint-plugin-jinja"
]
}
```

Eslint does not pick up the `preprocessor` because this [line](https://github.com/eslint/eslint/blob/master/lib/cli-engine.js#L171) is assuming CommonJS exports. If I modify `lib/index.js` to export the `preprocessors`, then it is picked up.

Am I missing something? Happy to submit a PR with the change.