Added command to generate quartz in a whole world

I quite liked your plugin but wanted to be able to fill my entire nether with quartz and not just loaded chunks.
So I added that functionality to your code. I'm also checking for quartz in the chunk before starting the ore population so that chunks from the new version don't get filled twice.
Since I've seen that there are more people wanting that feature I decided to send you a pull request.
Do whatever you want with it. If you have any questions don't hesitate to ask.