Added appending to data file instead as a strecht goal

[simbs]

During the workshop I changed the code to be able to only append a new post to the data file instead of reading all of it, parsing, adding to the string and rewriting everything. However, the way I've done it means posts.json is no longer a true JSON file, so I don't think this version should replace the original method which allows to explain what JSON is.

I haven't added my solution to it, let me know if you think I should include it (and where!).