Request: Get pretty-print summary of available Cake Targets (eg. build.ps1 -ListTargets)

One of the challenges we have across our organization is that teams structure their cake files differently for their particular workflows. It's difficult as new comer to understand which targets do what you need. We ineffectively rely on team-lore, outdated readmes, or noisy slack channels for help.

It would helpful if we could add switch to get a pretty print summary of targets and their relationships.

Something like, this would be very helpful:

```
PS C:\git\myproj> .\build.ps1 -ListTargets

Full
|
--> Clean
|
--> NugetRestore
|
--> Build
|
--> Package
|
--> Publish
```

Start with the build.ps1 command entry point and trace how Cake discovers and runs targets. Determine how target relationships should be represented and rendered for -ListTargets; done means a readable target summary matching the requested example and covering the relevant command behavior.

Written by the indexing model from the issue text.