Changes to the `pages` variable

There's 2 commits for 2 different things related to the `pages` variable in the template:

- Ignoring files that start with an underscore (common practice in flat-file systems)
- Make custom meta headers available (the ones added using the `before_read_file_meta` hook, as explained in the docs)

So if you do

```php
public function before_read_file_meta(&$headers)
{
	$headers['layout'] = 'Layout';
}
```

You get `{{ page.layout }}` while iterating `pages` as well as in `{{ meta.layout }}`.