Feature request: Use scoped slots to enable "full customizations"

Hey all!

I'm currently struggling with customizing navigation and pagination "properly" (no button in button and more than just label replacements).

As I've seen (#78, #88, #34) several other people had the same issues. A great way to deal with the customization problem are scoped slots. You don't lose the default settings for people who want a great slider and are good to go but enable "full customization possibilities" for everyone who wants to do more with the carousel.

It'll require some major restructuring of the code, but I think it's worth it!