Fix issues with using a different/custom path for the public folder

Because of the hardcoded `_public/js/dontUseMe` path in the plugin, the `partials.js` was not being created when using a different value, other than `_public`, for the `config.path.public` parameter.

For example, having set 
 
    paths: {
      public: 'public'
    }

in the Brunch `config.js` file, the `partials.js` file is not created at all.

This fixes the issue by replacing the `_public` part of the hardcoded path, with the `config.path.public` parameter, from the Brunch config file.