Add ability to set custom logger (#48)

Before this commit, a custom logger could be set with the LogMessages
function. However, by using LogMessages not only is a custom logger set
but also all received and sent messages are logged. Use cases exist
where a custom logger is desired to log errors but not all messages.

conn_opt.go

@@ -102,3 +102,10 @@ func LogMessages(logger Logger) ConnOpt {
 		})(c)
 	}
 }
+
+// SetLogger sets the logger for the connection.
+func SetLogger(logger Logger) ConnOpt {
+	return func(c *Conn) {
+		c.logger = logger
+	}
+}