[MRG+1] Improve RANSACRegressor

Adding a new parameter to the RANSAC regressor. I am not sure why I did not include this in the original PR, this should definitely be added. It is not totally backwards compatible, but I am not sure how you typically handle these cases. Also, I would theoretically group all the `stop_*` parameters next to each other. Let me know, how you want me to change this.