Allow to collect multiple instances per host

(as inspired by https://github.com/powdahound/redis-collectd-plugin/blob/master/redis_info.py)

Author: Jan Kunzmann

README.md

@@ -32,11 +32,14 @@ You should then configure the MySQL plugin:
 			Port 3306 (default: 3306)
 			User "root" (default: root)
 			Password "xxxx" (default: empty)
+      Instance "" (default: empty instance identifier)
 			HeartbeatTable "percona.heartbeat" (if using pt-heartbeat to track slave lag)
 			Verbose false (default: false)
 		</Module>
 	</Plugin>
 
+Repeat the "Module" section for multiple instances
+
 ## Metrics
 
 ### MySQL Status