Add formatter option

[twolfson]

Sometimes we receive data in a format than when we want to save it to disk. It can be usually it piped into another grunt task but not always. We should add a new option for this:

curl: {
  src: {...},
  dest: ...,
  formatter: function (res, body, cb) {
    cb(JSON.stringify(body));
  }
}

This issue was created as a side effect of #10

[m4w]

This would be still very helpful. Please add this function.