Add a 'timber_output' filter

This will allow us to filter the output anyway we like, further controlling our markup.
For example this function will add `data-pjax` to all internal links using this filter:

```php
function add_pjax_internal_links( $output ) {

	// convert to array so we can skip the <head>
	$output_array = explode( '</head>', $output );

	// Add pjax-data to internal links
	$output_array[1] = str_replace( 'href="/', 'data-pjax href="/', $output_array[1] );
	$output_array[1] = str_replace( 'href="' . get_home_url(), 'data-pjax href="' . get_home_url(), $output_array[1] );

	// convert array back to string
	$output = implode( '', $output_array);

	return $output;

}
add_filter( 'timber_output', 'add_pjax_internal_links' );
```