Permalink format for categories & tags

[tricoos]

I have not yet found a way on how to change the URL format of the categories and tags. It's currently just using the urlencoded category name but I want to have pretty names (slugs) and I've already created a function for it.
Yet, after hours of searching I haven't found a way on how and where to change the category page structure, i.e. I can change the links like in category.html, but I of course have to change the name in such a way that Sculpin also generates them in the same way.

So instead of
/categories/My%20Category/index.html
I'd like to have
/categories/my-category/index.html

However, where can I change the filename that's being used to generate these files?