logging: '-V' cli option can blacklist/whitelist classes with short names

for example, '-V ni' will whitelist the 'Network' and 'Interface' classes
'-V ^ni' will blacklist those instead

electrum/network.py

@@ -224,6 +224,8 @@ class Network(Logger):
     servers, each connected socket is handled by an Interface() object.
     """
 
+    LOGGING_SHORTCUT = 'n'
+
     def __init__(self, config: SimpleConfig=None):
         global INSTANCE
         INSTANCE = self