Add support for labeled-response

It's just used to avoid mixing up messages coming from the server
so far.

lib/irc.js

@@ -544,3 +544,19 @@ export function findBatchByType(msg, type) {
 	}
 	return null;
 }
+
+export function getMessageLabel(msg) {
+	if (msg.tags.label) {
+		return msg.tags.label;
+	}
+
+	var batch = msg.batch;
+	while (batch) {
+		if (batch.tags.label) {
+			return batch.tags.label;
+		}
+		batch = batch.parent;
+	}
+
+	return null;
+}