add tag {{term-slug}} to be used in templates

I had to move the input out of the label, so I needed an ID to reference the label to. Therefore I added a new tag that uses the slug of the category as identifier.
This is just meant to be an example implementation. In my current project I use the slug in another case as well (adding a slug-specific class to the li), so having a slug-tag in general is not a bad idea.