Trouble rendering html
Hello,

Jade works fine but when I switch to html I get an error. I even downloaded your example and tried it. I am using node version 0.11.13. I would very much appreciate you pointing out where I am going wrong. It would help me get this site into production. I am including all the possible info below. Thank you very much!

The debug statement from console is:
koa-views render `index.html` with {"user":"John","session":{"views":11}} +267ms

The error message is:
 TypeError: undefined is not a function
      at Object.<anonymous> (/home/bobby/Dropbox/http/testviews/node_modules/koa-views/node_modules/co-views/node_modules/co-render/index.js:33:5) at next /home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:99:21) at Object.<anonymous> (/home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:56:5) at next  (/home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:99:21) at Object.<anonymous> (/home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:56:5) at next (/home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:99:21) at Object.<anonymous> (/home/bobby/Dropbox/http/testviews/node_modules/koa/node_modules/co/index.js:56:5) at Server.<anonymous> (/home/bobby/Dropbox/http/testviews/node_modules/koa/lib/application.js:123:8)
      at Server.EventEmitter.emit (events.js:110:17)
      at HTTPParser.parserOnIncoming [as onIncoming](_http_server.js:504:12)

index.js:

``` javascript
var session = require('koa-session');
var router = require('koa-route');
var views = require('koa-views');
var koa = require('koa');
var app = koa();
/**
 * Session.
 */
app.keys = ['secrets'];
app.use(session());
/**
 * Setup views.
 */
app.use(views(__dirname, {
  default: 'html'
}));
app.use(function* (next) {
  var n = this.session.views || 0;
  this.session.views = ++n;
  this.locals = {
    session: this.session
  };
  yield this.render('index', {
    user: 'John'
  });
});
app.listen(3000);
console.log('app running on port 3000');
```

index.html

``` html
<!DOCTYPE html>
<html lang="en">
  <head>
    <title>koa views</title>
    <style>
      body {
        font-family: Helvetica;
        text-align: center;
      }
      h1 {
        padding-top: 300px;
        font-weight: 200;
        color: #333;
        font-size: 6em;
      }
      h2 {
        color: #666;
      }
    </style>
  </head>
  <body>
    <h1>koa ♥</h1>
    <h2>John has viewed this site  times.</h2>
  </body>
</html>
```
