Add sneak#prompt configuration

Well, that's it. It's not very elegant, but it's working fine and it's not breaking any test. I haven't used Vader before, but I hope it's a start :).

If I can improve this to get merged, just tell me.
I was using a minimal vimrc, but somehow I managed to put the `let ...` after loading the plugin, that's why it wasn't working.

Thanks for the information, I am an avid reader :B