ability to add your own custom handlers

[Almenon]

in https://github.com/Almenon/AREPL-backend/blob/master/python/customHandlers.py I have handlers for some objects like datetime and regex. When JSONpickle encodes these objects it uses the custom handlers. The end result is a different set of properties display, so you can provide a more human-friendly representation of the object.

The downside to this is that there are literally infinite possible variations of objects, and I can only cover the most common ones. Dynamically importing a python file of the user's choice would allow them to add custom handlers for objects of their choosing.

The alternative is that they have to submit a pull request to this repo. It's a lot easier on my end to merge a pull request than do all that dev work so until (if) arepl becomes popular this is going straight to the trashbin. I mean, the backlog.