Support post-dependencies

Issue #1818 was about reverse dependencies and optional dependencies. However, the way they were implemented doesn't support the use case I need them for. Using the example code from that issue:

Task("A");
Task("B").IsDependentOn("A");
Task("C").IsDependentOn("B").IsDependeeOf("D");
Task("D").IsDependeeOf("E");

RunTarget("D");

I would have expected the task traversal

`D` -> `E`

or changing the final line to run target `C`,

`A` -> `B` -> `C` -> `D` -> `E`

That is, I personally was expecting a sort of `finally` block for tasks, where `.IsDependeeOf(task)` is equivalent to `.Finally(() => {RunTask(task);})`, just like `.IsDependentOn(task)` could be approximated by including `RunTask(task)` in the Does block.

Additionally, pull request #1819 did not add the parameter `required` to the extension methods `IsDependentOn` and `IsDependeeOf`.

Read issue #1818 and pull request #1819 first, then inspect the IsDependentOn and IsDependeeOf extension methods and the task traversal behavior described in the examples. Clarify the required-parameter API and the intended post-dependency ordering before determining the relevant implementation and tests; done means both traversal examples behave as specified.

Written by the indexing model from the issue text.