Support more flexible interactive builds

I started this discussion in the `empire/#stacker` slack channel yesterday, and it sounds like multiple teams have thought about various ways to allow for interactive builds, w/ stacker running somewhere other than on a privileged-IAM user's machine.

Why are people stated for still running interactive builds from their machine for critical stacks?
* Need fine-grained visibility into the CFN resources being changed as a final sanity check
* Security - the current setup allows for mandated MFA to run

Various ideas were discussed to achieve parts of the requirements that I have in mind:
* Stacker communicates changesets to a lambda function, w/ the expectation that changesets get executed out of band by an IAM user that has the necessary access level. Stacker would probably poll for a change to the changeset (execute or delete) and would then resume processing the temporarily paused DAG plan.
* More generic "approval plugins" that, instead of doing IO on STDOUT/STDIN, it would delegate to a python class, specified in the stacker config.