Add custom backoff setter (#176)

Users should have the capability to customize the backoff pattern and accordingly adjust it within the retry mechanism.

retry/retry_example_test.go

@@ -51,6 +51,35 @@ func ExampleRetryWithLinearBackoff() {
 	// 3
 }
 
+type ExampleCustomBackoffStrategy struct {
+	interval time.Duration
+}
+
+func (c *ExampleCustomBackoffStrategy) CalculateInterval() time.Duration {
+	return c.interval + 1
+}
+
+func ExampleRetryWithCustomBackoff() {
+	number := 0
+	increaseNumber := func() error {
+		number++
+		if number == 3 {
+			return nil
+		}
+		return errors.New("error occurs")
+	}
+
+	err := Retry(increaseNumber, RetryWithCustomBackoff(&ExampleCustomBackoffStrategy{interval: time.Microsecond * 50}))
+	if err != nil {
+		return
+	}
+
+	fmt.Println(number)
+
+	// Output:
+	// 3
+}
+
 func ExampleRetryWithExponentialWithJitterBackoff() {
 	number := 0
 	increaseNumber := func() error {