Add `moduleGenerator` option to set the module name based on filePath

This pattern is broadly used in rake pipeline filters as the [minispade filter](https://github.com/wycats/rake-pipeline-web-filters/blob/master/lib/rake-pipeline-web-filters/minispade_filter.rb). 

It allows to use the filter with more different project structures or build process.

```js
var es6Options = { moduleGenerator: function(filePath) {
                    return filePath.replace(tmpFolder+'/', '')
                                   .replace('lib/','')
                                   .replace(/.js$/, '')
                                   .replace(/\/main$/, '');
                  }, 
                  anonymous: false };
 
app = es6Filter(app, es6Options); 
```