Add game step-tree capability

[lifengbo]

I think the WGO need add a step-tree capability include all brances, or is this functionality present somewhere? the step-tree like below:

• 1
  • 1-1
  • 1-2
  • 1-3
  • 1-4
  • 1-4-1
  • 1-4-2
• 2
  • 2-1
  • 2-2
  • 2-3
• 3
• 4
• 5

the picture like this:https://raw.githubusercontent.com/lifengbo/swift/master/chapter-Text/step-tree.png