Support for resource maps

Hey David, 

We're using just copy to solve a problem staging some css resources from the ace editor. Ace wants to fetch its own stylesheets and expects them to be co-located with the javascript. 

I modified the just-copy plugin to support this use case by allowing you to specify both a source and a destination, rather than just a source. 

You can add it to your config with something like the following: 

  justCopy:
    paths:[
      {
        src: "stylesheets/vendor/ace"
        dest: "javascripts/vendor/ace"
      }
    ]