Proposal: Add RunBefore/RunAfter methods.

[patriksvensson]

I propose we add two new methods to Cake tasks:

• RunBefore(string task)
• RunAfter(string task)

By introducing the RunAfter and RunBefore, we add more flexible ways of composing the task graph.

Changes

This would mean that the meaning of IsDependentOn have to change as well. The IsDependentOn method is now just a way of telling Cake that the task should run after another task (something that RunAfter would replace).

The Dependent part should be respected, and if the dependent task was skipped for any reason, the task with the dependency should fail. This will introduce a slight semantic change, but I think it won't have a big impact since this is how it's used today (but not enforced by Cake).

Task("A)
  .WithCriteria(false);

Task("B")
  .IsDependentOn("A");

// This will fail since 
// task A was skipped.
RunTarget("B");