Register Blade extensions for javascript and stylesheet tags

As discussed in Issue #6, users of Blade can use them like

```
<!DOCTYPE html>
<html>
<head>
  @stylesheets()
  @javascripts('application', ['data-requires' => 'foobar'])
</head>
<body>
<h1>Hello World!</h1>
</body>
</html>
```

Blade compiles this to

```
<!DOCTYPE html>
<html>
<head>
  <?php echo stylesheet_link_tag(); ?>
  <?php echo javascript_include_tag('application', ['data-requires' => 'foobar']); ?>
</head>
<body>
<h1>Hello World!</h1>
</body>
</html>
```