Allow user to pick which JDKs he wants to see

_This is one of the ideas for future development I have. No final assignment yet, just a subject to talk about it and possibly bring some good ideas together. I am looking forward to hear your feedback or thoughts._

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Right now, demos from all JDKs are being loaded in [Main.java](https://github.com/AloisSeckar/demos-java/blob/master/src/main/java/org/javademos/Main.java#L48). We only recommend in a comment, that user can comment-out some of the sets. But this is not very practical and user-friendly.

I would like to think about a way of passing in list of numbers and we will then show only the JDKs that were picked. I have no strong opinion about how to realize this yet...