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.
`let g:sneak#prompt='>>>'` will work if you do it in your .vimrc. This is because `get(g:, 'sneak#prompt', '>')` doesn't run until after your .vimrc is loaded (because Vim doesn't load plugins until after it loads your .vimrc).

> I can't put let g:sneak#opt.prompt = '>>> ' from command line

I lock `g:sneak#opt` to achieve some degree of immutability. The options map is meant to be read-only. It will be initialized with the `g:sneak#prompt` value in your .vimrc, if any.