Allow JSON serializer to be overridden

[markfinger]

It's probably worth allowing custom json serializers to be easily added.

The change should be pretty simple, see

python-react/react/render_server.py

Line 27 in 1bad117

serialized_props = json.dumps(props, cls=JSONEncoder)

Basically, RenderServer.render should accept a json_encoder kwarg, that defaults to the current one.

It's also probably worth explicitly exposing the default encoder as a RenderServerEncoder subclass, to make subclassing easier for downstream users.

[markfinger]

See also: #77