Enable credential passing for client

The client works great for me, I love how intuitive it is to use. Since my mqtt server requires authentication and I saw no builtin way to use it I added it this way to the client startup.

In paho one calls _client.set_username_pw after the client is instantiated but with the context manager setting up the context fails. Because one never enterst the context one never gets a chance to pass the credentials to the client.

asyncio_mqtt/client.py

@@ -12,7 +12,7 @@
 
 
 class Client:
-    def __init__(self, hostname, port=1883, *, logger=None):
+    def __init__(self, hostname, port=1883, *, username=None, password=None, logger=None):
         self._hostname = hostname
         self._port = port
         self._loop = asyncio.get_event_loop()
@@ -38,6 +38,9 @@ def __init__(self, hostname, port=1883, *, logger=None):
         if logger is None:
             logger = MQTT_LOGGER
         self._client.enable_logger(logger)
+
+        if username is not None and password is not None:
+            self._client.username_pw_set(username=username, password=password)
 
     async def connect(self):
         self._client.connect(self._hostname, self._port, 60)