Allow setting new languages without editing source code

[u0xaa55]

It would be great if we can set up new languages through options or a dedicated function rather than editing the source code.

That way datepickk can be registered as-is as a dependency into npm's package.json. By preventing others from having to edit your project you avoid duplication of your code and allow them to subscribe for future releases of your project without having to readapt/breaking things.