Page links api

Currently, page links are only loaded when passed to `TimberPost::get_content` or `TimberPost::content` explicitly. It would be nice to have a simple (and logicless) way of querying [Page-Links](http://codex.wordpress.org/Styling_Page-Links) (ie. multi-page content). So I added `TimberPost::get_paged_content()` and the shorthand `TimberPost::paged_content`. This way a theme author can decide to add paged content in the loop without having to worry about logic.

This API makes sense to me, what do you think?

```
{# page.twig #}
...
{{post.paged_content}}
```

Also in the PR: unit tests for the original pagination behaviour + the added methods.