[suggestion] adding code playgrounds

[unrooted]

It's a suggestion for adding code playgrounds to let everyone play with different codes. Let's say, repl.it links to the specific code to let the user play with it.
The project may seem a bit dead, tho I've almost done Scala and Kotlin (both of them) chapter locally, doing it in really free time as the mega-side project. I think that adding code playgrounds may add some fresh air to the project. Even I can do it, maybe when I'll add Scala and Kotlin chapters, what do you think?