Add support for custom json serializer

[dizballanze]

Added support for custom JSON serializers.
This allows to use more efficient JSON serializers like orjson or ujson

[coveralls]

Coverage remained the same at 91.34% when pulling a3feebc on dizballanze:master into 39f0ee6 on GehirnInc:master.

[yosida95]

@dizballanze Thank you for your contribution!

I think supporting custom JSON implementation is a good idea. But I also think passing loads and dumps every calls of #decode and #encode is troublesome and the users want custom JSON implementation don't switch between the standard JSON and the custom JSON for the each calls.

So, defining json_loads = json.loads and json_dumps = json.dumps as a class variable on JWT and overriding them by an instance variable on the constructer is better. How do you think?