WIP extend nav and breadcrumb badge support and whitespace reduction

[PanderMusubi]

The result of this PR is a huge reduction of whitespace in navigation items and breadcrumb items.

This supports the development of https://github.com/helloflask/bootstrap-flask/issues/336 and https://github.com/helloflask/bootstrap-flask/issues/191

I would like to suggest dropping the underscore of the _badge parameter in render_nav_item, but that will break backward compatibility. However, the other badge parameters will also not need one.

Additional question, the badge is currently shown as white text on white background. How to fix this?

After these questions have been answered, documentation and tests will be added.