Using custom document or cookie object on CookieStorage initialization

[imcrazytwkr]

First and foremost, thank you for writing this library. I'm currently migrating from a legacy and deprecated cookie-monster to it and wanted to ask if it is possible to add support for passing custom document or cookie object as an argument (or a config parameter) to a constructor.

It proves to be very useful for unit testing as you can pass mock cookie object and verify the component's results without the need to monkey-patch Node's global namespace. Currently I solve it with inheriting CookieStorage class and overriding the ._getCookie() and ._setCookie() methods during testing but it doesn't seem like a clean solution to me. If you are interested in implementing this feature, I can provide a pull request.