Install shell completion as an optional command

parser.go

@@ -77,6 +77,9 @@ func (a *App) buildRoutingSnapshot() *routingSnapshot {
 		flagAliases:    a.flagAliases,
 		discoveryFlags: a.discoveryFlags,
 	}
+	if node := a.commands["completion"]; node != nil {
+		snapshot.completion = node.command.record.completion
+	}
 	for _, rule := range a.routingRules {
 		if rule.record.kind != commandAliasRule {
 			continue