Update example to include a withoutSlack option (#96)

In #90 @twelsh-aw found a bug in a new implementation. This turned out
to be caused by us mocking time.

Since time mocking will always have some risks this diff proposes
to expand the `examples` we're using - since they use "real" time
they should be good enough to cover most basic cases like #90.

In particular:
- we add a "withoutSlack" test so that the exact case reported in #90
  doesn't happen. No slack option seems common enough to add it.
- updates "withSlack" example to actually show how slack operates.
  Due to non-even execution times I'm forced to round the time a bit.

Possible issues:
- test stability: I re-run the test a 1000 times without issues - the
  timing seems to be stable.
- test duration: in total we're extending the examples by 5ms, which
  shouldn't be human noticeable.

example_test.go

@@ -7,11 +7,38 @@ import (
 	"go.uber.org/ratelimit"
 )
 
-func Example() {
-	rl := ratelimit.New(100) // per second
+func Example_default() {
+	rl := ratelimit.New(100) // per second, some slack.
+
+	rl.Take()                         // Initialize.
+	time.Sleep(time.Millisecond * 45) // Let some time pass.
 
 	prev := time.Now()
 	for i := 0; i < 10; i++ {
+		now := rl.Take()
+		if i > 0 {
+			fmt.Println(i, now.Sub(prev).Round(time.Millisecond*2))
+		}
+		prev = now
+	}
+
+	// Output:
+	// 1 0s
+	// 2 0s
+	// 3 0s
+	// 4 4ms
+	// 5 10ms
+	// 6 10ms
+	// 7 10ms
+	// 8 10ms
+	// 9 10ms
+}
+
+func Example_withoutSlack() {
+	rl := ratelimit.New(100, ratelimit.WithoutSlack) // per second, no slack.
+
+	prev := time.Now()
+	for i := 0; i < 6; i++ {
 		now := rl.Take()
 		if i > 0 {
 			fmt.Println(i, now.Sub(prev))
@@ -25,8 +52,4 @@ func Example() {
 	// 3 10ms
 	// 4 10ms
 	// 5 10ms
-	// 6 10ms
-	// 7 10ms
-	// 8 10ms
-	// 9 10ms
 }