Add type documentation and fix the example spider

With any API in Python, it is always better to *specify* what types the methods expect and what types they return. This is a kindness to the users of that API, and also enables [modern tooling](https://www.jetbrains.com/pycharm/) to catch errors.

Also, the [example/example/spiders/custom_kafka_spider.py](https://github.com/dfdeshom/scrapy-kafka/blob/2cbd027d/example/example/spiders/custom_kafka_spider.py#L3) was incorrect, which hinders adoption of the project.