Allow users to tag links as 'non-navigational'

[smee]

This is especially useful in react components that use links internally to trigger callback code rather than as a real link. In these instances we don't want to add duplicated history entries.

After some more investigation, I'm not convinced, that this is such a good idea. My main motivation has been the react-bootstrap wrappers, which use links for active, clickable elements. It is not possible to add custom attributes to all of these elements. I will withdraw this pull request.