feature(stack): Added a 'finally' target

[phoebesimon]

What Let users add a target that will execute after the success or failure of the 'deploy' target

How stack.js: Let user define a 'finally' target that will run after success or failure of the 'deploy' target README.md: Updated readme to describe the 'finally' target

Why Currently, if a deploy fails, it means that we won't clean up the suppressions we put in place at the start of the deploy. This lets us perform cleanup actions regardless of failure

[jirwin]

LGTM. Can you add a finally task to the example stack that is include?

[phoebesimon]

@jirwin added in 349f890

[jirwin]

+1