Allow option to skip "no-code" demos

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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This is related to #268 - but instead of "linking" demos, there are also quite a lot JEPs with no actual code to run, just comments about how this feature is meant to work and why. This is perfectly fine, because there are a lot of peformance or maintenance JEPs introduced, which cannot be turned into a code example anyway. But again - there are a lot of JEPs and printing out all of them may produce really long outputs.

So as in #268, user should be able to skip those that have code: false parameter in respective /resources/JDKxxInfo.json file.