Add an `output_filename` option to allow for custom output filenames.

Customize the output filename. Function is passed the source filename and returns an output filename. If no value is returned, default behavior is used.

```json
  options: {
    output_filename: function(file) {
      // only rename en.po. All other files use default behavior.
      if (/en\.po/.test(file)) {
        return 'english.json';
      }
    }
  }
```