Allow OAuth2 client id to be configured locally (#168)

Author: satterly

alertaclient/config.py

@@ -9,6 +9,7 @@
     'profile': None,
     'endpoint': 'http://localhost:8080',
     'key': '',
+    'client_id': None,
     'username': None,
     'password': None,
     'timezone': 'Europe/London',