Sources/output.vim improvement

In #687 I have mentioned this, and I am sorry your solution may not be what I am looking for.
In order to make my self clear, I made a short gif:

![test](https://cloud.githubusercontent.com/assets/568457/3944735/a9ef8cac-2617-11e4-9c58-875fc904f1c5.gif)

Instead of asking input every time, I think it's better to do it only once.