Support rescheduling jobs

[JohnRoesler]

What does this do?

• Adds support for rescheduling jobs using the following syntax:

  func ExampleScheduler_Update() {
  	s := gocron.NewScheduler(time.UTC)
  	j, _ := s.Every("1s").Do(func() {})
  
  	time.Sleep(10 * time.Second)
  	j, _ = s.Job(j).Every("10m").Update()
  
  	time.Sleep(30 * time.Minute)
  	_, _ = s.Job(j).Every(1).Day().At("02:00").Update()
  }

Which issue(s) does this PR fix/relate to?

#72

List any changes that modify/break current functionality

Have you included tests for your changes?

Did you document any new/modified functionality?

• Updated example_test.go
• Updated README.md

Notes