Add Category entity for blog articles
Relate categories to articles with 1:n
Set options for categories in ModuleOptions class
Show category name when available
Create route to show articles based on category name
Add Category CRUD system with Controller/Service/Form
Set cascading on-delete mappings

Remove all blog posts when a blog is deleted. However, set the
category to NULL when a category is removed.
Add hydration strategy for the Category field of Articles
Allow to unset categories by accepting null values
Add a slug to the category and use this as lookup
Select category in article form using the repository
Merge pull request #33 from Soflomo/feature/blog-categories

[WiP] Feature/blog categories