Feat/add borger dk

[AJDERS]

This PR depends on #9 .

Adds the borger.dk-scrape. borger.dk, is not a simple tree structure of categories/subcategories/articles, but rather:

• A set of different adjacent navigation hierarchies. (Regular tree hierarchy / user-questionaires / javascript-toggle-bullshit)
• These hierarchies are navigating a single set of categories/subcategories.
• Each subcategory contains a set of nested articles.
• Each article is a text and a link, and the link navigates up, down and sideways in the category/subcategory/nested article hierarchy (and also to adjacent hierarchies)

To mitigate this we:

• Pick one navigation hierarchy (the regular tree hierarchy), and avoid the others.
• Process each category analogous to how sundhed.dk was scraped, with the following additions:
  • We make sure to only follow links downward or sideways within a category/subcategory, i.e. that we do not leave a category once we've entered it.
  • Since there are many ways to navigate to the same article we store which urls we've visited, to avoid parsing them twice.

[saattrupdan]

Just a small extra addition. Feel free to merge in any case.