Make cleaning white space on save an option

I really like using white-space mode. 

However I'm often working on an existing third-party code base and I don't like turning in a huge commit with only one line of *real* changes and the rest is whitespace cleanups.

Accordingly, what I really need is to visually see all the white-space,
but it not be cleaned up when the file is saved.  But only for the one project, I still want to auto-fix my own code.

To accomplish this I've added a prelude-clean-whitespace-on-save config.  The config's value is checked on save so it can be over-ridden by dir-locals. If someone has a better (briefer) name or other suggestions, I'll be happy to rework.

Thanks much for all the great work!