Do not rewrite user's snipets on update

[RB-Lab]

After extension update all my snippets was replaced with shipped with extension ones. That's wrong behaviour. Or maybe phrase "you can edit the existing javascript.json file" should be removed form extension description if that file should to be rewritten every time.

[zaggino]

We can't change this behaviour so correct way is to place javascript.json somewhere outside of the extension direcyory and link to it through settings dialog.

[zaggino]

This has been noted to the documentation in #38