Rel Attribute as select field

We are using the patch from #5 which does not apply against latest 8.x-1.x branch.
I rerolled that patch and added more allowed rel attributes from https://www.w3schools.com/tags/att_a_rel.asp

Just for people that are already using an older version of this patch.

How about making it an options list instead of a selector? That could allow multiple items to be added. Going a step further, it'd be useful to be able to control which of them can be used, e.g. if you only want to add an option for nofollow and don't need all of the rest.

Changing the field type="checkboxes" doesn't work.