Template Teamwork: Implement the ability to use vain with another template engine.

This PR implements the ability for vain to be used with another template engine, such a jade. In order to facilitate this functionality, a new method named `renderFileWith` was added to the mix, and the router we provide was updated to support searching for templates ending in multiple file extensions.

To use this with jade in an application, you would do something like the following in your app.js file:

```javascript
// Tell express to use jade wrapped with vain for .jade files.
app.engine('jade', vain.renderFileWith('jade'));
// Tell the vain router to only consider jade files as templates.
app.use('/', vain.router(app.get('views'), 'jade'));
```

closes #12