Cross linking issues/pr's between repositories doesn't display a selection box.

Hey,

Not sure if this is a bug or if it's just a lack of a feature, but if you cross link between repositories (e.g. octocat/Hello-World#339), it doesn't display a selection box that gives a preview of the issue like it does when you're linking locally (e.g. #114):

[kivikakk]

👋 It's a lack of a feature; please do reach out to our support team with this request — this repository is just for issues for the github-markup gem, which doesn't have anything to do with issue/PR autocompletion. The support team keeps track of feature requests and how many people are asking for them. Thanks! ❤️