Added more settings keys in order to integrate with Play Framework

I am currently trying to make a project that integrates the Play framework, ScalaJs and the ScalaJs Workbench. I have added a few more settings key to the workbench in order to make it more configurable:

I have added `reloadPrefix` in order to specify the location from which the javascript are reloaded.
 Eg: instead of reloading from `localhost:12345` reload from `localhost:9000` where the play server is running.

I have also added `javascriptUrlGenerator` that lets you specify how javascript file paths are converted into urls. For example I need to convert stuff like `scalajvm/target/scala-2.10/classes/public/javascripts/scalajs.js` into `assets/javascripts/scala.js`.