Support 3rd-party Rework plugins

Example:

```
var queryless = require('queryless');

module.exports = function(grunt) {
  grunt.initConfig({
    rework: {
      target: { files: { 'out.css', 'in.css' } },
      options: {
        use: [
          [queryless, 'screen and (min-width: 60em)']
        ]
      }
    }
  })
};
```