classification: add command-and-control classtype

Added new classtype 'command-and-control' to be used with more
general TROJAN/MALWARE categories to designate traffic between
infected machine and c2 server.

classification.config

@@ -75,3 +75,4 @@ config classification: pup-activity,Possibly Unwanted Program Detected,2
 config classification: credential-theft,Successful Credential Theft Detected,1
 config classification: social-engineering,Possible Social Engineering Attempted,2
 config classification: coin-mining,Crypto Currency Mining Activity Detected,2
+config classification: command-and-control,Malware Command and Control Activity Detected,1