Make confluent-kafka-python optional dep (#74)

setup.py

@@ -36,9 +36,9 @@ setup(
         'antlr4-python2-runtime==4.7.1',
         'statsd==3.2.1',
         'retrying==1.3.3',
-        'confluent-kafka==1.0.0'
     ],
     extras_require={
         ':python_version=="2.7"': ['typing>=3.6'],  # allow typehinting PY2
+        'kafka': ['confluent-kafka==1.0.0'],  # To use with Kafka source extractor
     },
 )