Changes the execution mode for `watch`, asking for user input
We've [observed](https://hackmd.io/-cK6aPhnTwiCiI7u6k0xug?both) that learners can get confused when they do get everything right, but they _still_ get errors... which come from the next exercise, no the one they just edited.
This PR changes it so they have to confirm they want to move forward by removing the `I AM NOT DONE` comment.
![Screenshot at 2019-11-11 15:13:39](https://user-images.githubusercontent.com/1636604/68593566-0abd3900-0496-11ea-9e9d-6c43b91bf21d.png)
* [ ] The particular string is of course subject to bikeshed.
### Alternatives/doubts
* The coolest solution I could imagine would involve a proc-macro attribute `#![ready(false)]` that they could edit once they're done, but it's a bit complicated to set up.
* For now I've put `I AM NOT DONE` everywhere, I think it's what make more sense.