add contentForHeader

in the layout file(s) add `contentForHeader`

```
<html>
<head>
....
{{ contentForHeader }}
</head>
...
```

it's useful if you want to load scripts or stylesheets from templates.

```
{%- header %}
{{ 'mystyle.css' | stylesheet_tag }}
{%- endheader %}
```

also its useful for `preload` to work properly.
