Losing the engine
Testing this out I have this code:

``` js
app.use(views('../views'), {
    html: 'underscore'
});

app.use(function *(next) {
    this.body = yield this.render('index', { name: 'koa' });
});
```

and grapped in the index.html file from the Underscore example. But when I open the page I get this error:

> TypeError: undefined is not a function
>     at Object.<anonymous> (/Users/thomas/CloudStation/Dropbox/www/webBlueprint/node_modules/koa-render/node_modules/co-views/node_modules/co-render/index.js:33:5)
>     at next (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:74:19)
>     at Object.<anonymous> (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:91:5)
>     at next (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:74:19)
>     at Object.<anonymous> (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:91:5)
>     at next (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:74:19)
>     at Object.<anonymous> (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:91:5)
>     at next (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:74:19)
>     at Object.<anonymous> (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:91:5)
>     at next (/Users/thomas/CloudStation/Dropbox/www/koala/node_modules/koa/node_modules/co/index.js:74:19)

I enable Debug and got this:

> <-- GET /
>   co-views views ../views {} +0ms
>   co-views render ../views/index.html {"name":"koa","engine":"html","cache":false} +0ms
>   co-render render ../views/index.html with {"name":"koa","engine":"html","cache":false} +0ms
>   --> GET / 200 2ms -

So somewhere the underscore engine is lost and the example doesn't work? Or am I doing something wrong?
