Inverse lookup for NiceUrls and contenttype-only routes

This adds two new functionalities to NiceUrls extension:

**ContentType-only routes:**

<pre>
pages:
  from:
    slug: p
  to:
    contenttypeslug: pages
    slug: ""
</pre>

**A filter for inverse lookup of internal links:**
`<a href="{{ previous.link | niceurl }}">Previous</a>`

Also did some refactoring.