Adding a --concurrency (-C) flag

concurrency.go

@@ -0,0 +1,25 @@
+package task
+
+func (e *Executor) acquireConcurrencyLimit() func() {
+	if e.concurrencySemaphore == nil {
+		return emptyFunc
+	}
+
+	e.concurrencySemaphore <- struct{}{}
+	return func() {
+		<-e.concurrencySemaphore
+	}
+}
+
+func (e *Executor) releaseConcurrencyLimit() func() {
+	if e.concurrencySemaphore == nil {
+		return emptyFunc
+	}
+
+	<-e.concurrencySemaphore
+	return func() {
+		e.concurrencySemaphore <- struct{}{}
+	}
+}
+
+func emptyFunc() {}