separate index.html into multiple files

[evykassirer]

If we want to make it easy to play around with the code and add new functionality, a good first change would be to separate the JavaScript from the html from the licence/header comments into multiple files (which would improve readability).

This would be a nice way to get acquainted with the code, so I'll look into it once my exams are done if no one else is interested in taking it on!