Add a capture to article include to use external-url

I tried using the `external-url` feature as described in the [docs for writing a linklog](http://octopress.org/docs/blogging/linklog/). 

It didn't work for me, so I looked into the includes and made this change for myself. It seems to work, although I'm not sure if it's in 100% alignment with the project goals or if there are other parts of Octopress that I am not properly accounting for. 

Anyway, for what it's worth...