Prevent clearing the CSS/JS cache via settings.

It would be quite helpful for boost (along with some other caching projects I'm sure) if the Drupal core had an option to not flush its own aggregated css/js folders. The generating functions md5's the filename so being worried about old css/js isn't a huge issue. Right now it's a huge pain to deal with #413908: Cache js/css files, allow for a "static" site; currently requiring an ugly core hack.