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 knew that this wasn't very clean.... but when I was testing my changes, I found that I can't put `let g:sneak#opt.prompt = '>>> '` from command line (from a minimal vimrc, it's the same error):

> Error detected while processing command line:
E741: Value is locked: g:sneak#opt.prompt='>>>'

That's why I decided to use that function, specially because it was needed in two parts within the `s:getnchars` function.