diffCopy task for comparing files before copying

Hello.

This is a task I wrote for personal use, and I found it comes very much in handy when combined with the excellent [grunt-newer](https://www.npmjs.org/package/grunt-newer). I'm not sure if the contrib plugin is the best place for this task, but since it's so dependent on the `copy` multitask and offers no real standalone functionality, I thought I'd offer it up as a Pull Request.

Any ideas on fleshing out the tests for this task? Right now just piggybacking on existing copy tests, with manual check of log output to see if the second pass is a no-op as expected.

Thanks for the wonderful plugin!