how to define Customer filter in nunjucks ?
filter.js
```
import nunjucks from 'nunjucks';

let env = new nunjucks.Environment();
env.addFilter('sex', function(str){
    switch (str){
        case 1:
            return "男";
        default :
            return "女";
    }
});


module.exports = env;
module.exports = nunjucks;//or this


```

```
import filter from './filter';
app.use(views(__dirname + '/views', { options: {
    nunjucksEnv:filter,
    settings: {
        views: path.join(__dirname, 'views')
    }
},map: {html: 'nunjucks' }}))
```

but not validate