Add attributes to links

[jan-dh]

For tracking purposed I need the content editors to be able to add custom attributes to links. Classes, data-attributes, ... things like that. A string field, a list of items to select from, ... doesn't really matter, but this would be a huge improvement for the plugin. Can help out with testing etc. if needed.

The current setup (adding extra fields) is really awful from a AX/UX point of view.