FinallyTrees - Ability to execute code before ending the tree walker session

[TravisOnGit]

This needs a lot more thought, but here's the rough idea. Ability to define some code that will get executed before ending the tree walker session.

The code/Actions(?)/subroutines that get executed could be a growing list as the tree is walked.
Or perhaps they could be added to the ForgeAction definitions themselves.

This would be useful to ensure clean-up code gets executed, even on failure.