Added functionality to influence enabled extensions for candle and light

This allows loading of wix extensions, such as WixUtilExtension, which we need to register an event source
This was a bad pattern (my fault). We should [do this instead](https://github.com/opscode/omnibus/blob/6c9e6771deef7a12ee2cefff56d8c28dd3d75c10/spec/unit/packagers/deb_spec.rb#L31-L43).