Option to default open link in new window to on

[nickcobley]

It would be nice if when allowing the user the choice if the link will open a new windown/tab or not, to be able to set its default state. In our case the majority of the links would want to be opened in an external window, which relies on the user remembering to tick this box every time.

Giving us this option would mean we can enforce a much more consistent experience.

This is similar #82 although much more limited in scope.