WIP: allow to add any links to seo page

Mostly to cover cases like 

```
<link rel="prev" href="http://www.example.com/article?story=abc&page=1" />
<link rel="next" href="http://www.example.com/article?story=abc&page=3" />
<link rel=”publisher” href=”https://plus.google.com/[YOUR BUSINESS G+ PROFILE HERE]“/>
```

Changes look slightly cumbersome to support BC. Let me know if you would like me to proceed with changes like that and if you want to do a BC break. 

Tasks:
[ ] add twig helpers to render all link elements