[Proposal] add onlyStale option to prevent unnecessary re-minifying

I think that an `onlyStale` option would be useful, because in large projects, calling the cssmin task often results in the minification of many files that haven't been modified. In which case, the time spent minifying them is wasted. By adding the optional (off by default) `onlyStale` option, cssmin can now intelligently decide whether a destination file needs to be re-combined and minified based on the mtimes of its composing sources.

I see a particular benefit to this option for those using grunt-contrib-watch (where unnecessarily reminifying a project of many files on each file save causes refresh times to be slower).