feature: HTMLSerializer - custom fallback for .serialize

[jvanasco]

HTMLSerializer.serialize has a chained if/else that ends with this:

        else:
            self.serializeError(token["data"])

I propose supporting a custom fallback method that would allow subclasses to invoke the benefit of this serializer's method to deal with unknown/custom token types. this would allow custom parsers/tree-adapters the ability to override how something is rendered by changing the token type and registering a custom handler... which still leveraging the core serializer.

for example:

        else:
            if self._customTypes and (type in self._customTypes):
                 yield self.self._customTypes[type](token)
            else:
                self.serializeError(token["data"])