Add option to specify an output dir

Hi,

Thanks for sharing this project, really useful.

I've got a complex build with a lot of different artifacts, a few different processing steps, so it's hard to use simple globbing patterns (*.js) to manage them. It makes my config a lot simpler if grunt-rev supports the ability to specify an output directory... hence the pull request.

Tests and linting are passing, I think I've stuck to the same style, and I've added an example to the README. Let me know what you think, any suggested edits, etc.