Some uses may want to optionally set a target on custom links. Any chance this feature could be added?
I've made a modification to this plugin on the website I'm using it in to allow a target to be set on custom urls, though I've just made it a text field with a placeholder of "_blank" as a hint. But I could you could change this to a checkbox if it's only going to be same window or _blank.
Also, rather than set target attribute, the checkbox could set a class of "external" on the link through which to hook some jQuery that actually opens in a new window, for accessibility purposes.
If you think something like this is a good idea, but don't have the time to implement it, I don't mind forking and issuing a pull request if you want to include any change I made back into the official plugin.