Add Frontend::static

Add a `static` method to `Bolt\Controllers\Frontend` for displaying recordless content.

Proposed usage:

```
home:
  path: /
  defaults: { _controller: 'Bolt\Controllers\Frontend::static', _template: 'home' }
```

```
public static function static(Silex\Application $app, $_template) {
  return $app['render']->render($_template . '.twig');
}
```
Edit: ok, obviously `static` is a keyword, so maybe `template` instead.