Implemented meta twig helper

Helper functions to generate `<link rel="alternate" ...` and seo `<meta ...` html tags.

__Example:__

```twig
{% autoescape false %}
<!-- Seo -->
{{ meta_seo(extension, content) }}

<!-- Alternate -->
{{ meta_alternate(urls) }}
{% endautoescape %}
```

__Tasks:__

- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

Related PR: https://github.com/sulu-cmf/sulu-standard/pull/374

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | TODO