Add ability to configure pool growth strategy

examples/task_group/go.mod

@@ -0,0 +1,9 @@
+module github.com/alitto/pond/examples/task_group
+
+go 1.14
+
+require (
+	github.com/alitto/pond v1.3.0
+)
+
+replace github.com/alitto/pond => ../../

examples/task_group/task_group.go

@@ -0,0 +1,28 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/alitto/pond"
+)
+
+func main() {
+
+	// Create a pool
+	pool := pond.New(10, 1000)
+	defer pool.StopAndWait()
+
+	// Create a task group
+	group := pool.Group()
+
+	// Submit a group of related tasks
+	for i := 0; i < 20; i++ {
+		n := i
+		group.Submit(func() {
+			fmt.Printf("Running group task #%d\n", n)
+		})
+	}
+
+	// Wait for all tasks in the group to complete
+	group.Wait()
+}