For most use cases I imagine you'd want to be able to specify this at the level of the link, which is why I tagged this with inclusion.
{{foo|link;target:new}}
(Weird. I typed stuff in here when adding the card and it did not take.)
All of the target options in HTML: http://www.w3schools.com/tags/att_link_target.asp
You could go with target:_blank which would line up with the HTML syntax. On the other hand, Wagn markup is intended to be useable by people who don't even know HTML, and "_blank" (both "blank" and the underscore, which we use differently in other contexts) is pretty opaque imho.
Just wanted to chime in that I'd like to be able to not just do this on a link level but be able to set a global that all outside links open in a new window. I personally don't see why I'd want to have people navigating away from my site some of the time.
cool. not a bad idea. Thanks, Lora.
+discussed in support tickets
+relevant user stories