hooks not hacks

I've included a new approach for encapsulating the differences between the java visualizer and your original one. It has 8 hooks, there's also a less hacky version of the left-right slider. The java visualizer is using the hooks API as follows:

https://github.com/daveagp/java_visualize/blob/master/pytutor-customizations.js

Let me know if you think this approach could be incorporated in the mainstream! It would be great since then I could keep it merged continuously with very little work. I hope the approach could be useful for other languages too.