Enduring state for actions?

[pnmadelaine]

#36 sparked a discussion about offering some way of keeping state between actions' runs.

To quote myself from this PR, I am not yet sold on the feature. I am more inclined to keep things as simple as possible at first and wait for people to come up with scenarios that we are interested in handling that require the feature.

Thus I open this issue to track interest and potential use cases.