Modernize the app

[Bam92]

I came across this project at random when looking for a tool to create a database schema.

I tried to look at the code and I've noticed two issues:

• there's no tests (unit or whatever)
• old js
  So, I wish we can make the app a bit more modern by adding tests and adopting modern JavaScript.

[ondras]

* So, I wish we can make the app a bit more modern by adding tests and adopting modern JavaScript.

Feel free! I am too busy these days, so re-factoring the app is out of my scope. But I would probably accept a PR that adjusts the code to modern JavaScript, whatever that currently means. Just do not introduce any other dependencies.

(Unfortunately, I do not think this would actually present any kind of improvements for users, but whatever.)