Ability to define a task which is always called after an action chain workflow finishes (either fails or succeeds)

[Kami]

It would be useful if user could define a step which is always called when a workflow finishes and / or fails. You can think of it as "finally" in Python and other programming languages.

Similar end result can be achieved by adding "on-failure" to every step, but defining this function once is nicer.

Here is an example from a workflow I'm currently working on:

[zhenglisai]

this feature is needed, the join in workflow not work very well when some task no need to execute.