Prevent evil nbsp from being invisible

Not seeing these (Non-breaking spaces) can be problematic, since you won't know what broke your code on the day that one of them sneaks in.

Here's a few other common characters people use for this: https://github.com/search?utf8=%E2%9C%93&q=nbsp%3A+extension%3A.vimrc&type=Code&ref=searchresults
What do you think about using `·` for this since it's a space?